Planar lightwave circuit for conditioning tunable laser output

ABSTRACT

A planar lightwave circuit (PLC) module for conditioning light output from a tunable laser designed to generate light at a target wavelength. The PLC module has a substrate; a primary waveguide embedded in said substrate, said primary waveguide having an input end for receiving light from the tunable laser and an output end for outputting said light; and at least a first secondary waveguide embedded in said substrate, said first secondary waveguide receiving a first portion of said light from the tunable laser. A filter having a passband centered on the target wavelength is coupled to an output of the first secondary waveguide to receive said first portion of light, and generates a signal related to the intensity of said first portion of light in the passband centered on the target wavelength. This may be used by a processor and associated laser control circuitry for wavelength locking purposes.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This nonprovisional U.S. national application, filed under 35U.S.C. § 111(a), claims, under 37 C.F.R. § 1.78(a)(3), the benefit ofthe filing date of provisional U.S. national application No. 60/272,623,filed under 35 U.S.C. § 111(b) and accorded a filing date of Mar. 1,2001, the entirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to devices that emitelectromagnetic radiation and, in particular, to wavelength monitoringand locking for a semiconductor laser.

[0004] 2. Description of the Related Art

[0005] The following descriptions and examples are not admitted to beprior art by virtue of their inclusion within this section.

[0006] Lasers have a wide range of industrial and scientific uses. Thereare several types of lasers, including gas lasers, solid-state lasers,liquid (dye) lasers, and free electron lasers. Semiconductor lasers arealso in use. In semiconductor lasers, electromagnetic waves areamplified in a semiconductor superlattice structure. Semiconductorlasers may be diode lasers (bipolar) or non-diode lasers such as quantumcascade (QC) lasers (unipolar). Semiconductor lasers are used for avariety of applications and can be built with different structures andsemiconductor materials, such as gallium arsenide (GaAs).

[0007] The use of semiconductor lasers for forming a source of opticalenergy is attractive for a number of reasons. Semiconductor lasers havea relatively small volume and consume a small amount of power ascompared to conventional laser devices. Further, semiconductor laserscan be fabricated as monolithic devices, which do not require acombination of a resonant cavity with external mirrors and otherstructures to generate a coherent output laser beam.

[0008] A semiconductor laser typically comprises an active (opticalgain) region sandwiched between two mirrors (reflectors or reflectiveplanes). There is typically a small difference in reflectivity betweenthe two mirrors, one of which (typically, the reflective plane havinglower reflectivity) serves as the “exit” mirror. The area between thereflective planes is often referred to as the resonator, or theFabry-Perot resonance cavity in some cases. The active region is locatedwithin the resonant cavity. When the active region is pumped with anappropriate pumping energy, it produces photons, some of which resonateand build up to form coherent light in the resonant cavity formed by thetwo mirrors. A portion of the coherent light built up in the resonatingcavity formed by the active region and top and bottom mirrors passesthrough the exit mirror as the output laser beam.

[0009] Various forms of pumping energy may be utilized to cause theactive region to begin to emit photons and to achieve gain. For example,semiconductor lasers of various types may be electrically pumped (EP)(by a DC or alternating current), or pumped in other ways, such as byoptical pumping (OP) or electron beam pumping. In an EP VCSEL, forexample, an electrical potential difference is typically applied acrossthe active region (via top and bottom electrical contacts provided aboveand below the active region). As a result of the potential applied, apumping current flows through the active region, i.e. charge carriers(electrons and holes) are injected from opposite directions into theactive region where recombination of electron and holes occurs. Thereare two kinds of recombination events, i.e. radiative and non-radiative,concurrently happening in the active region. When radiativerecombination occurs, a photon is emitted with the same energy as thedifference in energy between the hole and electron energy states. Someof those photons travel in a direction perpendicular to the reflectorsof the laser. As a result of the ensuing reflections, the photons cantravel through the active region multiple times.

[0010] Stimulated emission occurs when radiative recombination of anelectron-hole pair is stimulated by interaction with a photon. Inparticular, stimulated emission occurs when a photon with an energyequal to the difference between an electron's energy and a lower energyinteracts with the electron. In this case, the photon stimulates theelectron to fall into the lower energy state, thereby emitting a secondphoton. The second photon will have the same energy and frequency as theoriginal photon, and will also be in phase with the original photon.Thus, when the photons produced by spontaneous electron transitioninteract with other high energy state electrons, stimulated emission canoccur so that two photons with identical characteristics are present.(Viewed as waves, the atom emits a wave having twice the amplitude asthat of the original photon interacting with the atom.) If a sufficientamount of radiative recombinations are stimulated by photons, the numberof photons traveling between the reflectors tends to increase, givingrise to amplification of light and lasing. The result is that coherentlight builds up in the resonant cavity formed by the two mirrors, aportion of which passes through the exit mirror as the output laserbeam.

[0011] Semiconductor lasers may be edge-emitting lasers orsurface-emitting lasers (SELs). Edge-emitting semiconductor lasersoutput their radiation parallel to the wafer surface, while in SELs, theradiation output is perpendicular to the wafer surface. One type of SELis the vertical-cavity surface-emitting laser (VCSEL). The “vertical”direction in a VCSEL is the direction perpendicular to the plane of thesubstrate on which the constituent layers are deposited or epitaxiallygrown, with “up” being typically defined as the direction of epitaxialgrowth. In some designs, the output laser beam is emitted out of the topside, in which case the top mirror is the exit mirror. In other designs,the laser beam is emitted from the bottom side, in which case the bottommirror is the exit mirror.

[0012] VCSELs have many attractive features compared to edge-emittinglasers, such as low threshold current, single longitudinal mode, acircular output beam profile, a smaller divergence angle, andscalability to monolithic laser arrays. The shorter cavity resonator ofthe VCSEL provides for better longitudinal mode selectivity, and hencenarrower linewidths. Additionally, because the output is perpendicularto the wafer surface, it is possible to test fabricated VCSELs on thewafer before extensive packaging is done, in contrast to edge-emittinglasers, which must be cut from the wafer to test the laser. Also,because the cavity resonator of the VCSEL is perpendicular to thelayers, there is no need for the cleaving operation common toedge-emitting lasers.

[0013] The VCSEL structure usually consists of an active (optical gain)region sandwiched between two mirrors, such as distributed Braggreflector (DBR) mirrors. Both EP and OP VCSEL designs are possible. Thetwo mirrors may be referred to as a top DBR and a bottom DBR. Becausethe optical gain is low in a vertical cavity design, the reflectorsrequire a high reflectivity in order to achieve a sufficient level offeedback for the device to laser.

[0014] DBRs are typically formed of multiple pairs of layers referred toas mirror pairs. DBRs are sometimes referred to as mirror stacks. Thepairs of layers are formed of a material system generally consisting oftwo materials having different indices of refraction and being easilylattice matched to the other portions of the VCSEL, to permit epitaxialfabrication techniques. The layers of the DBR are quarter-waveoptical-thickness (QWOT) layers of alternating high and low refractiveindices, where each mirror pair contains one high and one low refractiveindex QWOT layer. The number of mirror pairs per stack may range from20-40 pairs to achieve a high percentage of reflectivity, depending onthe difference between the refractive indices of the layers. A largernumber of mirror pairs increases the percentage of reflected light(reflectivity).

[0015] The DBR mirrors of a typical VCSEL can be constructed fromdielectric (insulating) or semiconductor layers (or a combination ofboth, including metal mirror sections). The difference between therefractive indices of the layers of the mirror pairs can be higher indielectric DBRs, generally imparting higher reflectivity to dielectricDBRs than to semiconductor DBRs for the same number of mirror pairs andoverall thickness. Conversely, in a dielectric DBR, a smaller number ofmirror pairs can achieve the same reflectivity as a larger number in asemiconductor DBR. However, it is sometimes necessary or desirable touse semiconductor DBRs, despite their lower reflectivity/greaterthickness, to conduct current, for example (e.g., in an EP VCSEL).Semiconductor DBRs also have higher thermal (heat) conductivity than dodielectric DBRs, making them more desirable for heat-removal purposes,other things being equal. Semiconductor DBRs may also be preferred formanufacturing reasons (e.g., a thicker DBR may be needed for support) orfabrication reasons (e.g., an epitaxial, i.e. semiconductor, DBR may beneeded if other epitaxial layers need to be grown on top of the DBR).

[0016] When properly designed, these mirror pairs will cause a desiredreflectivity at the laser wavelength. Typically in a VCSEL, the mirrorsare designed so that the bottom DBR mirror (i.e. the one interposedbetween the substrate material and the active region) has nearly 100%reflectivity, while the top (exit) DBR mirror has a reflectivity thatmay be 98%-99.5% (depending on the details of the laser design). Thepartially reflective top (exit) mirror passes a portion of the coherentlight built up in the resonating cavity formed by the active region andtop and bottom mirrors. Of course, as noted above, in other designs, thebottom mirror may serve as the exit mirror and the top mirror has thehigher reflectivity. VCSELs, DBRs, and related matters are discussed infurther detail in Vertical-Cavity Surface-Emitting Lasers: Design,Fabrication, Characterization, and Applications, eds. Carl W. Wilmsen,Henryk Temkin & Larry A. Coldren (Cambridge: Cambridge University Press,1999).

[0017] In standard VCSELs, the active region and top and bottom mirrorsare monolithically fabricated on a substrate. A variant on the standardVCSEL, an external cavity VCSEL, or vertical-external-cavitysurface-emitting laser (VECSEL), is also in use. In this case, theactive region and bottom mirror are monolithically fabricated on asubstrate, while the top mirror is mounted externally, some distance(typically very small) above the active region. VECSELs are described inJ. Sandusky & S. Brueck, “A CW External-Cavity Surface-emitting Laser,”IEEE Photon. Techn. Lett. 8, 313-315 (1996). The term VCSEL may be usedto refer to both standard (monolithic) VCSELs and external-cavity VCSELS(VECSELs).

[0018] VCSEL characteristics are capable of extensive modeling andmanipulation. Sarzala et al., “Carrier Diffusion Inside Active Regionsof Gain-Guided Vertical-Cavity Surface-Emitting Lasers,” IEEEProc.—Optoelectonics, vol.144, no. 6,p. 421-24, December 1997, Langleyet al., “Effect of Optical Feedback on the Noise Properties of VerticalCavity Surface Emitting Lasers,” IEEE Proc.—Optoelectonics, vol. 144,no. 1, p. 34-38, February 1997, Ha et al., “Polarisation Anisotropy inAsymmetric Oxide Aperture VCSELs,” Electronics Letters, vol. 34, no. 14,July 1998.

[0019] Semiconductor lasers such as VCSELs and edge-emitting lasers areused in a variety of applications. In some applications, e.g.,telecommunications and spectroscopy among others, the output laser lightis modulated to achieve the objective of the system. Modulation consistsof modifying a characteristic of the laser output, e.g., the amplitude,frequency, or phase. In the case of telecommunications, the modulationsare patterned to correspond to information. The laser may be externallymodulated, or directly modulated. When the radiation of the output laserbeam is detected after it has traveled to another point, the modulationsindicate the information that was encoded at the transmitter/modulatorend. A typical telecommunications system uses optical fiber to guide theradiation from the modulation (or emission) point to the detectionpoint. Long wavelength (1.3 μm to 1.55 μm) VCSELs, for example, are ofgreat interest in the optical telecommunications industry because of theminimum fiber dispersion at 1310 nm and the minimum fiber loss at 1.55μm (1550 nm).

[0020] It is important to be able to monitor, and sometimes control, thewavelength of the emitted laser radiation in some applications. Intelecommunications applications, for example, the emitted laserradiation of a given semiconductor laser has a precise wavelength, asspecified, for example, by the ITU grid. For example, the ITU gridspecifies lasing wavelength of 1.55 μm (and other closely spacedwavelengths). These ITU grid wavelengths are used in telecommunicationsapplications such as coarse and dense wavelength-division multiplexing(CWDM and DWDM). In WDM, typically used in optical fiber communications,two or more optical (e.g. laser) signals having different wavelengthsare simultaneously transmitted in the same direction over one fiber, andthen are separated by wavelength at the distant end.

[0021] The use of wavelength-division multiplexed communications systemshas led to additional equipment. For example, devices for demultiplexingthe wavelengths include the disclosure of U.S. Pat. No. 5,894,535(1999), Lemoff et al., “Optical Waveguide Device for WavelengthDemultiplexing and Waveguide Crossing.” That patent discloses a deviceincluding a zigzag patterned dielectric channel waveguide structure thatguides a WDM signal through a zigzag path. At particular vertices of thepath optical filters selectively transmit and reflect wavelengths oflight. The light output of the device separates wavelength of light byoutput position. As another example, U.S. Pat. No. 5,673,129 (1997),Mizrahi, “WDM Optical Communication Systems with Wavelength StabilizedOptical Selectors” discloses a system that receives a portion of a WDMsignal with a Bragg grating having one high reflectivity wavelengthband. Based on the signal that is reflected from the grating, awavelength parameter of the Bragg grating is modified, resulting in achange in the high reflectivity wavelength band. A system basingfeedback on the signal transmitted by the grating is also disclosed.U.S. Pat. No. 6,111,681 (2000), Mizrahi et al., “WDM OpticalCommunication Systems with Wavelength Stabilized Optical Selectors”contains the same disclosure as U.S. Pat. No. 5,673,129.

[0022] Systems that adjust the output wavelength of a laser have alsobeen proposed. For example, U.S. Pat. No. 5,943,152, Mizrahi et al.,“Laser Wavelength Control Device” discusses a system that couples anin-fiber Bragg grating to the output of a laser. Based on either thetransmissivity or reflectance of the grating, a microprocessorcontinually adjusts the wavelength of the laser output. As anotherexample, U.S. Pat. No. 5,875,273, Mizrahi et al., “Laser WavelengthControl Under Direct Modulation” discusses a system using a filter withparticular transmission characteristics as a function of wavelength. Inparticular, the filter includes a transmissivity minimum withtransmissivity maximums for both a greater and lesser wavelength, whichcan also be described as a high reflectivity wavelength band. The filteris coupled to a laser and a control circuit adjusts the laser'swavelength characteristics based on measurement of both reflected andtransmitted light from the filter. As another example, U.S. Pat. No.6,067,181, Mizrahi, “Laser Locking and Self Filtering Device” discussesa laser system with an optical transfer element and a Bragg grating. Theentire output of the laser is coupled to the Bragg grating via thetransfer element. The light reflected from the Bragg grating isoutputted while the light transmitted through the Bragg grating isdetected to generate a signal that is used to control the laser. U.S.Pat. No. 6,125,128 (2000), Mizrahi, “Laser Output Locking and SelfFiltering Device” contains substantially the same disclosure as U.S.Pat. No. 6,067,181.

[0023] It can be difficult to ensure that a given laser is lasing at thedesired wavelength, and to control or tune the laser to emit atdifferent wavelengths. For example, VCSELs can have a wavelengthsignificantly dependent on drive current (or some other tuningparameter), and can be thus said to be “tunable”. In general, a tunablelaser such as a tunable VCSEL is a laser having an output wavelengthcorresponding to a selectable tuning parameter. Some approaches used inattempts to tune various types of lasers are described in B. Pezeshki,“New Approaches to Laser Tuning,” Optics & Photonics News, 34-38 (May2001). These include, in addition to varying the pumping or drivecurrent, temperature variation, combination of multiple lasers havingdifferent wavelengths on a single chip, and movement of micromechanicalcomponents.

[0024] However, while tunability is desired in some applications, it cangive rise to undesired variation in lasing wavelength. Additionally,even lasers that initially have a fixed or stable wavelength can havewavelength drift over time, as the device ages. It can be important tobe able to determine that desired wavelength, or wavelength “lock,” hasbeen lost. Information about deviation of the current wavelength fromsome benchmark or target wavelength can be useful for diagnostic orlocking purposes, for example.

[0025] There is, therefore, a need for methods and devices to permitmonitoring, stabilizing, selecting, and controlling the lasingwavelength of semiconductor lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Other features and advantages of the invention will becomeapparent upon study of the following description, taken in conjunctionwith the attached FIGS. 1-16.

[0027]FIG. 1 is a chart of reflectance as a function of wavelength for areflector.

[0028]FIG. 2A is a chart of reflectance as a function of wavelength fora reflector constructed in accordance with FIG. 2C.

[0029]FIG. 2B is a chart of reflectance as a function of wavelength fora reflector constructed in accordance with FIG. 2C.

[0030]FIG. 2C is a diagram of a reflector.

[0031]FIG. 3A is a chart of reflectance as a function of wavelength fora reflector constructed in accordance with FIG. 3C.

[0032]FIG. 3B is a chart of reflectance as a function of wavelength fora reflector constructed in accordance with FIG. 3C.

[0033]FIG. 3C is a diagram of a reflector.

[0034]FIG. 4A is a chart of reflectance as a function of wavelength fora reflector constructed in accordance with FIG. 4C.

[0035]FIG. 4B is a chart of reflectance as a function of wavelength fora reflector constructed in accordance with FIG. 4C.

[0036]FIG. 4C is a diagram of a reflector.

[0037]FIG. 5A is a chart of reflectance as a function of wavelength fora reflector constructed in accordance with FIG. 5B.

[0038]FIG. 5B is a diagram of a reflector.

[0039]FIG. 6A is a chart of reflectance as a function of wavelength fora reflector constructed in accordance with FIG. 6D.

[0040]FIG. 6B is a chart of reflectance as a function of wavelength fora first implementation of a reflector constructed in accordance withFIG. 6D.

[0041]FIG. 6C is a chart of reflectance as a function of wavelength fora first implementation of a reflector constructed in accordance withFIG. 6D.

[0042]FIG. 6D is a diagram of a reflector.

[0043]FIG. 7A is a chart of reflectance as a function of wavelength fora second implementation of a reflector constructed in accordance withFIG. 6D.

[0044]FIG. 7B is a chart of reflectance as a function of wavelength fora second implementation of a reflector constructed in accordance withFIG. 6D.

[0045]FIG. 8 is a diagram of a system for monitoring a laser.

[0046]FIG. 9 is a diagram of a system for monitoring a laser.

[0047]FIG. 10A is a diagram of a system for monitoring an externalcavity laser.

[0048]FIG. 10B is a diagram of an external cavity laser.

[0049]FIG. 11 is a diagram of a vertical cavity surface emitting laser.

[0050]FIG. 12 is a diagram of a system for monitoring a vertical cavitysurface emitting laser.

[0051]FIG. 13 is a diagram of a system for monitoring a laser.

[0052]FIG. 14 is a flowchart depicting a method of configuring a tunablelaser.

[0053]FIG. 15 is a flowchart depicting a method of changing wavelengthband in a tunable laser.

[0054]FIG. 16 is a diagram of zigzag waveguide device-based wavelengthlocking apparatus.

[0055] While the present invention is susceptible to variousmodifications and alternative forms, specific embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE INVENTION

[0056] Referring now to the drawings, the details of preferredembodiments of the invention are schematically illustrated.

[0057] Multiple Reflectivity Band Reflector

[0058] A reflector is fabricated having a structure that gives rise tomultiple high, and narrow, reflectivity bands. Such a reflector may bereferred to as a multiple reflectivity-band reflector (MRBR). In anembodiment, the reflectivity bands of the MRBR are spaced alongwavelengths of the ITU grid, e.g. around 1550 nm. In another embodiment,additional reflectivity bands of the MRBR are spaced around 1310 nm. Inan embodiment, the MRBR is a distributed dielectric multilayer stackreflector.

[0059] Each reflectivity band has a peak reflectivity and an associatedpeak wavelength, and is separated from adjacent reflectivity bands bywavelength bands which may be referred to as “troughs”. The peaks of thereflectivity bands, as well as the minima (bottom tips) of the troughsbetween bands (i.e. transmissivity peaks), of an MRBR, have a certainreflectivity profile (envelope). In the present application, the term“reflectivity profile” refers to the shape of the function intersectingthe peaks of bands (or the trough minima of troughs between bands), fora given mirror's reflectance versus wavelength function (reflectancecharacteristic). The reflectivity profile of the peaks of reflectancebands may be referred to as the peak profile, while the trough profilerefers to the reflectivity profile of the trough minima. Thereflectivity profile may also be referred to as the “envelope function”or profile for the peaks or trough minima of a given reflectancecharacteristic. In some embodiments, the reflectivity profile (e.g., ofthe reflectivity band peaks) is substantially constant over a givenwavelength range (as described in further detail below with reference toFIGS. 1-4). In other embodiments, the peak profile varies as a functionof wavelength (as described in further detail below with reference toFIG. 5).

[0060] Depending on the embodiment, either all, some, or none of thereflectivity bands have a reflectivity sufficient to give rise tolasing. Reflectivity bands that do have sufficient reflectivity topermit lasing may be referred to as lasing threshold reflectivity bands.

[0061] Each reflectivity band has a peak reflectivity and a narrowwavelength band encompassing the peak, in which band all thereflectivities are above some reflectivity threshold. Between adjacentpeaks or bands are troughs in which the reflectivity is below thisthreshold reflectivity.

[0062] In an embodiment, the reflectivity of a plurality of reflectivitybands of the MRBR is high enough to give rise to lasing in a VCSEL ortwo-section VECSEL configuration employing the MRBR. In this case, thereflectivity bands are lasing threshold reflectivity bands. Each lasingthreshold reflectivity band has a reflectivity, in a very narrowwavelength band, above a certain minimum or lasing thresholdreflectivity. This threshold reflectivity is high enough so that thereis net gain at that wavelength (i.e., the gain exceeds the mirror loss).The lasing threshold reflectivity band thus denotes the narrow range ofwavelengths centered about the peak reflectivity wavelength for thereflectivity band and for which the reflectivity is above some lasingthreshold reflectivity.

[0063] Lasing occurs when, during a round trip of photons through thecavity, the number of photons added due to stimulated emission is atleast equal to the number lost internally and at the edges. A lowerreflectivity for a given reflectivity band leads to more photons beinglost at the “edge” of the cavity formed by the MRBR. Thus, lasing ispossible where gain (determined by the gain spectrum of the activeregion) is greater than loss (determined in part by the reflectivity ofthe DBR mirrors), and where the phase difference of a round trip oflight within the optical cavity is zero. A wavelength where the phasedifference of a round trip of light within the optical cavity is zeromay be referred to herein as a cavity mode or a zero-phase differencewavelength. The precise cavity modes for a given laser are determined bythe physical distance between the mirrors and by the reflectivity andphase shifting characteristics of the DBR mirrors, and by the indices ofrefraction of various layers of material within the laser structure. Thecavity modes can be shifted by adjusting some indices of refractionwithin the laser cavity, by changing the physical cavity length, orother techniques.

[0064] This minimum reflectivity which is sufficient for lasing in agiven laser structure may be referred to as the lasing thresholdreflectivity and may be, for example, 99.5%. Each reflectivity peak thatis above the lasing threshold reflectivity is within an associatedlasing threshold reflectivity band. Between any two adjacent lasingthreshold reflectivity bands the reflectivity is necessarily below thelasing threshold reflectivity. As noted above, the troughs are portionsof the mirror's reflectivity profile between any two adjacentreflectivity bands. As will be appreciated, a trough between twoadjacent lasing threshold reflectivity bands will have a reflectivitylower than the lasing threshold reflectivity over the entire trough, andfor virtually all of the trough (except for the portion immediatelyadjacent the bottom portions of the adjacent reflectivity bands) lowerthan a second threshold reflectivity (e.g., 99.3%) which is lower thanthe lasing threshold reflectivity.

[0065] The reflectivity bands of the MRBR are preferably sufficientlynarrow. The reflectivity band may be characterized in terms ofnarrowness with respect to the wavelength band covered by a givenreflectivity, e.g. the first threshold reflectivity, or other specifiedhigh reflectivity (e.g., 99%). The wavelength range covered by areflectivity band above some first threshold reflectivity may be denotedAKTR (where “TR” stands for first “threshold reflectivity”), and therange covered with greater than 99% reflectivity may be denoted Δλ₉₉.Where the first threshold reflectivity is a lasing thresholdreflectivity (i.e., where the reflectivity band is a lasing thresholdreflectivity band), the denotation Δλ_(LT) may be employed instead ofΔλ_(TR), where “LT” stands for “lasing threshold.” The reflectivity bandmay also described as having a certain width at a reflectivity a certainpercentage (e.g., 3%) below its peak reflectivity. For example, in anembodiment, each reflectivity band has a width of less than 1 nm at areflectivity of 3% less than its peak reflectivity. Alternatively, in anembodiment, a given reflectivity band may be described as having Δλ₉₉less than or equal to some width (e.g., 1 nm, or 0.1 nm, etc.).

[0066] The particular lasing threshold reflectivity depends uponcharacteristics of the laser cavity such as the reflectivity of theother edge (of the cavity—whether a cleaved edge or DBR or other type ofreflector, e.g.), the degree to which the active region absorbsparticular wavelengths of light, and the degree to which atoms in theactive region are stimulated to emit additions photons at particularwavelengths of light (i.e., the gain). For example, if the bottom mirrorof a VECSEL has very high reflectivity, then lasing thresholdreflectivity for the other mirror (e.g., the MRBR) is lower. Conversely,a higher lasing threshold reflectivity is needed for the top or externalmirror (e.g., the MRBR) given a lower reflectivity bottom cavity mirror.Lasing can occur where there is a cavity mode, at a wavelength lyingwithin a given reflectivity band (i.e., at a cavity mode wavelength atwhich the reflectivity of the reflectivity band exceeds the lasingthreshold reflectivity).

[0067]FIG. 1 plots the reflectance versus wavelength characteristics foran exemplary uniform reflectivity profile MRBR in accordance with thepresent invention. The reflectance is determined with unpolarized lightimpacting the MRBR perpendicularly through air. The reference wavelengthis 2000 nm. The “reference wavelength” term specifies the wavelengthused to determine the QWOT for the layers of the MRBR; i.e., they have athickness that is equal to a quarter of the reference wavelength in thatmaterial. The MRBR with the reflectance characteristics shown in FIG. 1has a total thickness of approximately 212.4 μm, and may be fabricatedon a suitable substrate, such as glass or InP. As will be appreciated,various multilayer stack structures and materials may be utilized toresult in reflectivity bands of various wavelengths, spacing, and width(narrowness), and having different reflectivities for the peaks andtrough minima, and various constant or varying peak and trough minimaprofiles.

[0068] Referring now to FIGS. 2A-B, there are shown plots of thereflectance versus wavelength characteristics for another exemplary MRBRin accordance with the present invention, the structure of which isshown in FIG. 2C. This MRBR also has a substantially uniformreflectivity profile for at least a plurality of peaks. FIG. 2B graphs amuch smaller portion of both axes than is shown in FIG. 2A. For bothFIG. 2A and FIG. 2B, the reflectance is determined with unpolarizedlight impacting the MRBR perpendicularly through air. The MRBRreflectance characteristics shown in FIGS. 2A-B may be achieved byemploying an MRBR having a layer structure either substantially similarto or based on/derived from the structure shown in FIG. 2C and definedby the following Formula (1):

EABCD(ABC)⁶⁰ABCF(ABC)⁶⁰AB   (1)

[0069] where an exponent indicates that the preceding term is repeatedthat many times in succession, the letters “qw” denote quarter-waveoptical thickness (QWOT) (for a given reference wavelength), and thelayer symbols in the formula indicate the following materials andthicknesses: Symbol Material Thickness A Al₂O₃ 0.7500 qw B TiO₂ 0.7500qw C SiO₂ 0.7500 qw D Si 0.7500 qw E Al 1.2500 qw F Si 0.7500 qw

[0070] The reference wavelength is 1800 nm. For the simulations used togenerate the plots for the MRBRs of the present invention, the qwfigures were based on the following assumed indices of refraction andextinction coefficients: Material Refractive Index ExtinctionCoefficient Al₂O₃ 1.62 0 TiO₂ 2.3 0 SiO₂ 1.434 0 Si 3.4 0 Al 2.3 16.5

[0071] For example, the A layer has a quarter wavelength equal to thereference wavelength divided by both the refractive index of thematerial and by 4: (1800 mn/1.62)/4 =277.778 nm. Division by therefractive index accounts for the fact that the wavelength of lightdepends on the propagating medium. The A layer is therefore0.75*qw=0.75*277.778 nm=208.33 (208) nm thick. The same calculationproduces the thicknesses of the other layers: Symbol Material ThicknessA Al₂O₃ 208.33 nm B TiO₂ 146.74 nm C SiO₂ 235.36 nm D Si  99.26 nm E Al244.57 nm F Si  99.26 nm

[0072] As noted above, to obtain the desired MRBR reflectancecharacteristics (e.g., those shown in FIGS. 2A-B), an MRBR may beemployed having a layer structure either substantially similar to orbased on/derived from the structure defined Formula (1). For example, inan embodiment, to arrive at the actual MRBR layer structure to beemployed, the layer structure of Formula (1) is used as a startingpoint, to result in a modified layer structure similar to and/or basedon the initial layer structure specified in Formula (1). For example,the reflector structure may be optimized by varying the individual layerthicknesses so as to achieve the precise reflectance characteristicsdesired. Thus, although Formula (1) indicates many layers havingidentical thickness, after optimization the reflector utilized may havelayers of different thicknesses. Whether the MRBR employed has a layerstructure substantially similar to, or based on/derived from a structuredefined by a given formula, the MRBR may be said to have a layerstructure “based on”, “derived from,” or “corresponding to” the layerstructure defined by the formula.

[0073] In alternative embodiments or with other fabrication processes orsputtering devices, the refractive indices and extinction coefficientsof the layer materials employed may vary somewhat from those assumedabove. In this case, the qw figures might need to be adjusted, to takeinto account actual indices of refraction, to achieve a givenreflectivity characteristic. The qw parameter for each of the simulatedplots of FIGS. 1-7 is the QWOT for a given reference wavelength, denoted“reference” on the legend at the top of each of FIGS. 1-7. Thus, in someembodiments, due to optimization or other empirical adjustments, theformula itself may be adjusted or the implementation may be adjusted.For example, for Formula (1), layer A appears several times in theformula. Although nominally indicated as being an Al₂O₃ having athickness of 0.7500 qw (or 208.33 nm), this thickness of layer A may bevaried, either for all the A layers in the structure or only some ofthem.

[0074] Formula (1) specifies a 371-layer stack, where the leftmostsymbol denotes the layer(s) closest to the substrate. The substrate isnot shown in FIG. 2C. The reflector may be fabricated by any suitabletechnique, e.g., a dielectric coating technique such as such as ion beamassisted sputtering or magnetron sputtering on a suitable substrate,such as InP or glass. In some cases, the reflector need not use asupporting substrate, i.e. it has an “air” substrate.

[0075] The particular stack layer formula employed may be empiricallydetermined, e.g. with the aid of a suitably programmed computer (e.g.,running the TFCalc program available from Software Spectra, Inc., havinga web site with a www domain name of sspectra.com) which attempts tofind suitable layer structures, within certain constraints, to yield thedesired reflectivity band characteristics. The qw parameter may beselected for a reference wavelength greater than or less than the actualdesired wavelength range for the reflectivity bands. For example,reference wavelengths of 1800, 2000, 2200, and 795 nm were employed, insome cases, to achieve reflectivity bands in the 1550 nm range.

[0076] As noted above, alternate layer structures may be employed toachieve desired reflectivity band characteristics. Referring now toFIGS. 3A-B, there are shown plots of the reflectance versus wavelengthcharacteristics for another exemplary MRBR in accordance with thepresent invention and diagrammed in FIG. 3C. FIG. 3B graphs a muchsmaller portion of both axes than is shown in FIG. 3A. For both FIG. 3Aand FIG. 3B, the reflectance is determined with unpolarized lightimpacting the MRBR perpendicularly through air. The MRBR reflectancecharacteristics shown in FIGS. 3A-B may be achieved by employing an MRBRhaving a layer structure either substantially similar to or basedon/derived from the structure shown in FIG. 3C and defined by thefollowing formula:

EABCD(ABC)⁶⁰ABCF(ABC)⁶⁰AG   (2)

[0077] where the layer symbols in the formula indicate the followingmaterials and thicknesses: Symbol Material Thickness A Al₂O₃ 0.7500 qw BTiO₂ 0.7500 qw C SiO₂ 0.7500 qw D Si 0.7500 qw E Al 1.0000 qw F Si0.7500 qw G Si 0.7500 qw

[0078] The reference wavelength is 1800 nm. By performing the thicknesscalculation, the following thicknesses are obtained: Symbol MaterialThickness A Al₂O₃ 208.33 nm B TiO₂ 146.74 nm C SiO₂ 235.36 nm D Si 99.26 nm E Al 244.57 nm F Si  99.26 nm G Si  99.26 nm

[0079] This MRBR has a total thickness of approximately 72.75 μm. Inembodiments with different optical characteristics, resulting forexample from different deposition techniques, different measurementswill be appropriate. In one embodiment the MRBR is fabricated on an InPsubstrate.

[0080] As can be seen, the MRBRs of FIGS. 2 and 3 each have reflectivitybands closely spaced, around the 1550 ITU grid wavelengths, withreflectivities exceeding 99.98%. The troughs have lower reflectivities.In an embodiment, the MRBR comprises a plurality of reflectivity bands,each covering one of a contiguous set of 1550 ITU grid wavelengths,preferably where each reflectivity band has a peak reflectivity at orvery close to the particular ITU grid wavelength covered by thatreflectivity band. In other embodiments, the MRBR reflectivity bands maycover non-contiguous ITU grid wavelengths over a certain wavelengthrange (i.e., there are some ITU grid wavelengths not covered by areflectivity band, in which lasing/monitoring is possible only for thoseITU grid wavelengths for which there is a reflectivity band). In stillother embodiments, over a given wavelength range, there may be somereflectivity bands that lie between ITU grid-covering reflectivitybands, i.e. there are more reflectivity bands than necessary to coverthe ITU grid wavelengths in the range, in which case the “extra”reflectivity bands may be ignored or unused, depending on theapplication.

[0081] Referring now to FIGS. 4A-B, there are shown plots of thereflectance versus wavelength characteristics for another exemplary MRBRin accordance with the present invention and shown in FIG. 4C. FIG. 4Bgraphs a much smaller portion of the reflectivity axis than is shown inFIG. 4A. For both FIG. 4A and FIG. 4B, the reflectance is determinedwith unpolarized light impacting the MRBR perpendicularly through air.The MRBR reflectance characteristics achieved by the MRBR shown in FIGS.4A-B may be achieved by employing an MRBR having a layer structureeither substantially similar to or based on/derived from the structureshown in FIG. 4C and defined by the following formula:

ABC(ABCD)²(ABDC)⁶⁸(ABCD)  (3)

[0082] where the layer symbols in the formula indicate the followingmaterials and thicknesses: Symbol Material Thickness A Al₂O₃ 0.7500 qw BTiO₂ 0.7500 qw C SiO₂ 0.7500 qw D Si 1.0000 qw

[0083] The reference wavelength is 2200 nm. By performing the thicknesscalculation, the following thicknesses are obtained: Symbol MaterialThickness A Al₂O₃ 254.63 nm B TiO₂ 179.35 nm C SiO₂ 287.66 nm D Si161.75 nm

[0084] This MRBR may be fabricated on an InP substrate, in anembodiment.

[0085] As will be appreciated from the Figures, the MRBRs of FIGS. 1-4have multiple, relatively uniformly spaced reflectivity bands and peaks,where each reflectivity band has a reflectivity above the lasingthreshold reflectivity, separated by reflectivity troughs, in which thereflectivity for all wavelengths in the wavelength band of the troughare lower than the lasing threshold reflectivity. As noted above, theparticular lasing threshold reflectivity depends upon characteristics ofthe laser cavity such as the reflectivity of the other edge, the degreeto which the active region absorbs particular wavelengths of light, andthe degree to which atoms in the active region are stimulated to emitadditions photons at particular wavelengths of light (gain). Thesecharacteristics can be modified under some circumstances. For example,the degree to which the active region is electrically pumped affects theoptical gain (gain spectrum). In an embodiment, the lasing thresholdreflectivity for a plurality of reflectivity bands may be the lasingthreshold reflectivity with respect to the gain at or near its maximum,e.g., assuming a gain of full-width half-maximum (FWHM) or greater.

[0086] In the MRBR embodiments of FIGS. 1-4, the reflectivity peaks havea substantially uniform reflectivity profile (envelope), i.e. thereflectivities of the peaks (and thus of the reflectivity bands) aresubstantially the same, for at least a plurality of contiguousreflectivity bands. However, the reflectivity trough minima may varywith wavelength over the wavelength range of interest, in someembodiments, as illustrated in FIGS. 2A, 3A, and 4A, meaning that theenvelope function or profile for the reflectivity trough minima varieswith wavelength. For example, the profile for the reflectivity peaksshown in FIGS. 2A, 3A, 4A is substantially uniform over wavelength,while the profile for the reflectivity trough minima vary overwavelength in accordance with some function.

[0087] As will be appreciated, the MRBR of the present invention mayhave a variety of uses, including wavelength monitoring, selecting, andlocking, as described below. In an alternative embodiment, the MRBR ofthe present invention may be designed with a reflectivity band profilewhich is substantially constant, but in which the reflectivity bandshave a reflectivity not necessarily above any lasing threshold. Such anMRBR may be employed for wavelength monitoring and locking purposes, asdescribed below. When the reflectivity bands of interest have areflectivity above the lasing threshold reflectivity, the MRBR may alsobe employed as one of the mirrors of the laser cavity itself, asdescribed below.

[0088] Multiple Reflectivity Band Reflector with Varying ReflectivityBand Profile

[0089] In an alternative embodiment, there is provided a reflector witha reflectivity band profile that varies as a function of wavelength in aknown manner over a given wavelength range. For example, in anembodiment, the reflectivity band profile may vary monotonically over atuning range, so that each reflectivity band has a unique reflectivitypeak within the tuning range. In such an embodiment, the reflectivitybands need not all be above the lasing threshold reflectivity. Forexample, the reflectivity bands may have peak reflectances from 70% downto 10%, decreasing in appreciable increments (e.g., 2%, 5%, 10%) fromband to band. Such a varying profile reflector may be utilized for laserwavelength monitoring of wavelengths within a given wavelength range ofinterest (e.g., a tuning range of a tunable laser). In otherembodiments, the trough minima (tranmissivity peaks) may be used insteadof the reflectivity peaks. Moreover, because each reflectivity peak (ortrough minima/transmissivity peak, depending on the embodiment) of theMRBR within the wavelength range of interest has a unique reflectivity(at least with respect to its neighboring reflectivity peaks/troughminima), the current lasing wavelength can be determined withoutcounting, as described in further detail below.

[0090] In an etalon, two flat, partially reflective parallel reflectorsare typically separated by a parallel spacer. This gives rise to aFabry-Perot interferometer type effect in which the interference ofmultiple beams in the reflective cavity results in constructive anddestructive interference at certain wavelengths. The condition forconstructive interference is that the light forms a standing wavebetween the two mirrors, i.e. the optical distance between the twomirrors must equal an integral number of half wavelengths of theincident light. There will be transmission (a transmissivity peak orreflectivity trough minima) where there is constructive interference,and reflection otherwise. Thus, the interference in the cavity givesrise to a series of equally-spaced transmission/reflection peaks. Thedistance (in wavelength) between adjacent peaks is known as the freespectral range (FSR) or channel separation. The FSR is a function of thephysical mirror separation, i.e. the distance between the two reflectorsof the etalon.

[0091] In an embodiment, a varying reflectivity band profile reflectorin accordance with the invention is provided by a modified etalon typereflector having a pair of parallel reflectors, at least one of whichhas a reflectance that varies over the wavelength range of interest(e.g., the tuning range). The reflectors are separated by a spacer layeror air gap of a given thickness (cavity distance). The cavity distancebetween the reflectors determines the FSR and thus the channel spacing,and the combined reflectance characteristic of the reflectors provide anenvelope function for the reflectivity and/or transmissivity peaks.Preferably, the reflectors are substantially symmetrical and havesimilar reflectivity profiles over the tuning range, preferably amonotonically varying reflectance over the tuning range. Thus, a varyingreflectivity band profile reflector in accordance with an embodiment ofthe present invention will have a series of reflectivity bands withinthe tuning range, spaced in accordance with the distance between thereflectors, where the reflectivity peaks of these bands monotonicallyvary over this tuning range.

[0092] In an embodiment, the two reflectors of the varying reflectivityband profile reflector are quarter-wave stacks, e.g. dielectric DBR typestacks, separated by an intervening spacer layer. In an embodiment, asuitably adapted MRBR may of the present invention may be designedhaving a first quarter-wave reflector section, an intervening spacerlayer, and a second quarter-wave reflector section, where the thicknessof the spacer layer is selected to achieve the desired FSR, and thelayer thicknesses, materials, and number of mirror pairs of the tworeflectors are selected to achieve an overall reflectance envelope thatvaries in a desired manner (e.g., monotonically) over the a given tuningrange. For example, the thickness of the qw layers of a qw stack may beselected based on a given reference wavelength to have a substantiallyhigh and constant reflectance over a wavelength range encompassing thereference wavelength, but which gradually tapers off at the end of thiswavelength range. The reference wavelength may be selected so that thisgradual tapering section occurs over the tuning range. For example, fora tuning range around 1550 nm (e.g., from 1540 to 1580 nm), a referencewavelength of about 800 nm may be selected for a given material system,to result in two qw mirrors each having substantially high and constantreflectance at 800 nm, but already gradually decreasing in the 1540-1580nm range. The combined effect of such a varying reflectivity profile forthe two reflectors of the etalon will be to bound or form amonotonically decreasing envelope around the reflectivity bands in the1540-1580 nm range.

[0093] Referring now to FIG. 5A, there is shown a plot of thereflectance versus wavelength characteristics for an exemplary MRBR withvarying reflectivity band profile in accordance with the presentinvention and diagrammed in FIG. 5B. For FIG. 5A the reflectance isdetermined with unpolarized light impacting the MRBR perpendicularlythrough air. The MRBR reflectance characteristics shown in FIG. 5A maybe achieved by employing an MRBR having a layer structure eithersubstantially similar to or based on/derived from the structure shown inFIG. 5B and defined by the following formula:

(AB)¹⁰D(BA)¹⁰   (4)

[0094] where the layer symbols in the formula indicate the followingmaterials and thicknesses: Symbol Material Thickness A Al₂O₃ 1.5000 qw BTiO₂ 1.5000 qw D Si 6000.0 qw

[0095] The reference wavelength is 795 nm. By performing the thicknesscalculation, the following thicknesses are obtained: Symbol MaterialThickness A Al₂O₃ 184.03 nm B TiO₂ 129.62 nm D Si 350.74 μm

[0096] This MRBR may be fabricated with air on both sides, because theSi layer (D) is thick and strong enough to support the MRBR without anexternal substrate. As can be seen in FIG. 5A, over a wavelength rangeof 1540 to 1580 nm, the reflectivity bands (which are spacedsubstantially uniformly about 1 nm apart) monotonically decrease inreflectivity from about 70% to less than 10%. Layer D is a thick layerthat separates the symmetric qw reflectors (AB)¹⁰ and (BA)¹⁰, thusgiving rise to the etalon type effect and its associated reflectancecharacteristic. In particular, the thickness of layer D controls thepeak separation or FSR. Changing the thickness of layer D thereforechanges the channel spacing, and thus may be used to achieve a narroweror wider channel spacing, depending on the application. In anembodiment, the MRBR comprises two substantially symmetric reflectors,which may be quarter-wave stacks, separated by a distance, e.g. thethickness of layer D, which may be a material such as silicon, or air inother embodiments.

[0097] As will be appreciated, the MRBR with varying reflectivity bandprofile of the present invention may have a variety of applications,including wavelength monitoring, selecting, and locking, as describedbelow.

[0098] Single Lasing-Reflectivity Peak Reflector

[0099] In an alternative embodiment, a reflector may be designed with asingle, narrow, high-reflectivity band which is above the lasingthreshold reflectivity. This single reflectivity band reflector may bereferred to as an SRBR, or as a single lasing-reflectivity peakreflector. Like the MRBR, the SRBR is a distributed dielectricmultilayer stack reflector. All other reflectivities outside the singlereflectivity band, including other peaks and trough minima, are belowthis threshold. In an embodiment, this band is narrow and covers asingle defined wavelength of a defined ITU grid set of wavelengths(e.g., around 1.55 μm). The lasing threshold reflectivity is thereflectivity required for one of the mirrors of a VCSEL or VECSELemploying the SRBR as one of its mirrors, i.e. the reflectivity requiredof the mirror when it forms part of the laser cavity of a VCSEL, e.g. atleast 99.5%. Outside the single reflectivity band, all otherreflectivities of the reflector are less than the lasing threshold,preferably less than some second threshold below the first one, e.g.less than 97%.

[0100] Referring now to FIGS. 6A-C, there are shown plots of thereflectance versus wavelength characteristics for an exemplary SRBR, inaccordance with the present invention and diagrammed in FIG. 6D. ForFIGS. 6A-C the reflectance is determined with unpolarized lightimpacting the MRBR perpendicularly through air. The reflectancecharacteristics shown in FIGS. 6A-C may be achieved by employing an SRBRhaving a layer structure either substantially similar to or basedon/derived from the structure shown in FIG. 6D and defined by thefollowing formula:

DC(AF)⁴⁰(E)¹⁰⁰(AF)¹⁰⁰G   (5)

[0101] where the layer symbols in the formula indicate the followingmaterials and thicknesses: Symbol Material Thickness A Al₂O₃ 1.0000 qw CSiO₂ 0.7500 qw D Si 0.7500 qw E Si 1.0000 qw F SiO₂ 0.7500 qw G Al0.0100 qw

[0102] The reference wavelength is 1800 nm. By performing the thicknesscalculation, the following thicknesses are obtained: Symbol MaterialThickness A Al₂O₃ 277.78 nm C SiO₂ 235.36 nm D Si  99.26 nm E Si 132.35nm F SiO₂ 235.36 nm G Al  1.96 nm

[0103] In one embodiment, this SRBR is fabricated on a suitablesubstrate, such as glass or InP. As can be seen, e.g. in FIG. 6C, asingle reflectivity band is achieved having a peak reflectivity atapproximately 1515.8 nm, which is very narrow (i.e., its peak is about99.5% reflectivity and it covers, at 99% reflectivity, about 0.02 nm inwavelength, i.e. Δλ₉₉=0.02 nm). All other reflectivities of any otherpeaks (and therefore all trough minima) of the SRBR of FIG. 6 are lessthan 97%, i.e. less than a second threshold (97%) which is itself lessthan the lasing reflectivity threshold or reflectivity threshold (e.g.,99%) for the primary reflectivity band. In this case, if 99% is taken tobe the lasing threshold reflectivity, then Δλ₉₉=Δλ_(LT) =0.02 nm, andthis mirror has a single, narrow reflectivity band above the lasingthreshold, i.e. a lasing threshold reflectivity band encompassing asingle with that reflectivity peak. In this case, the SRBR has a lasingthreshold reflectivity band of about 0.02 nm in width, with areflectivity peak wavelength of about 1515.8 nm. Lasing is thus possibleat wavelengths approximately within this narrow lasing thresholdreflectivity band, if other conditions are met (e.g., the laser isadequately powered, the lasing threshold reflectivity band includes acavity mode, etc.). In this case, the reflectivity band can also bedescribed as having a width of 0.02 nm at a reflectivity 0.5% below itspeak reflectivity (99% is about 0.5% below 99.5%); or a width of farless than 1 nm at a reflectivity a 3% below its peak reflectivity (i.e.,at about 96.5%).

[0104] Referring now to FIGS. 7A-B, there are shown plots of thereflectance versus wavelength characteristics for an exemplary SRBR, inaccordance with the present invention. For FIGS. 7A-B the reflectance isdetermined with unpolarized light impacting the MRBR perpendicularlythrough air. The reflectance characteristics shown in FIGS. 7A-B may beachieved by employing an SRBR having a layer structure eithersubstantially similar to or based on/derived from the structure shown inFIG. 6D and defined by Formula (6), though different from the layerstructure that results in the reflectance characteristics of FIGS. 6A-Cin that the fewer quarter wavelength silicon layers, layer E, areincluded. FIG. 6D does not indicate the specific number of quarterwavelength silicon layers, layer E, other than indicating that there aremore than three. Formula (6) is shown below:

(DC)¹(AF)⁴⁰(E)⁴⁰(AF)¹⁰⁰G  (6)

[0105] where the layer symbols in the formula indicate the followingmaterials and thicknesses: Symbol Material Thickness A Al₂O₃ 1.0000 qw CSiO₂ 0.7500 qw D Si 0.7500 qw E Si 1.0000 qw F SiO₂ 0.7500 qw G Al0.0010 qw

[0106] The reference wavelength is 1800 nm. By performing the thicknesscalculation, the following thicknesses are obtained: Symbol MaterialThickness A Al₂O₃ 277.78 nm C SiO₂ 235.36 nm D Si  99.26 nm E Si 132.35nm F SiO₂ 235.36 nm G Al  1.96 nm

[0107] This SRBR may be fabricated on a suitable substrate, such asglass or InP. As can be seen, e.g. in FIG. 7B, a single reflectivityband is achieved with a peak reflectivity wavelength of approximately1517 nm, which is highly reflective and very narrow (i.e., its peak isabout 99.4% reflectivity and Δλ₉₉ is about 0.12 nm). All otherreflectivities of the SRBR are less than 97%.

[0108] As will be appreciated, in various embodiments the SRBR of thepresent invention has a variety of uses, including wavelengthmonitoring, selecting, and locking, as described below.

[0109] In an alternative embodiment, the SRBR has one primaryreflectivity band and peak, having a reflectivity which is higher thanany other reflectivity peaks or troughs of the SRBR outside thatreflectivity band, but which is not above the lasing thresholdreflectivity. Such a reflector can be employed, for example, forwavelength monitoring purposes, but would preferably not be employed asone of the mirrors of a VCSEL laser cavity.

[0110] In the embodiments described above, the SRBR and MRBR of thepresent invention are distributed dielectric multilayer stackstructures, i.e. they are reflectors that comprise a plurality of layersof dielectric material arranged in parallel so as to provide the desiredreflectivity profile. In alternative embodiments, other materials may beemployed for some or all of the layers, such as semiconductor materials.

[0111] Multiple Reflectivity Band Reflector for Laser WavelengthMonitoring

[0112] In an embodiment, the MRBR of the present invention (e.g., theembodiments illustrated in FIGS. 1-5 above) is employed in a laserapparatus for laser wavelength monitoring purposes, in accordance withan embodiment of the invention. As used herein, “monitoring” includesmonitoring, controlling, tuning, selecting, and locking the outputlasing wavelength of a given semiconductor laser with the use of thereflectors of the present invention. The MRBR is part of the lasercavity itself, in some embodiments, or is used outside the cavity formonitoring and locking purposes in other embodiments.

[0113] Referring now to FIG. 8, there is shown an embodiment of thesemiconductor laser with MRBR wavelength monitoring of the presentinvention. As illustrated in FIG. 8, a tunable edge-emitting laser 810has an exit mirror 814 (with AR coating) and an HR (high reflectivity,but less than 100%) mirror 816, which define the laser cavity 812. In anembodiment, the laser 810 can emit over a wavelength range coveringseveral ITU wavelengths, under the control of a variable tuningparameter (e.g. temperature, gain, current, etc.). One or moremonitoring photodiodes, e.g. 826 a, 826 b, detect some of the cavitylight 822 passing through the HR mirror 816.

[0114] An MRBR 824 covering the tuning range of the laser is used as afilter, to monitor the lasing wavelength and channel. In an embodiment,the MRBR 824 is a coating on a light-receiving surface of one of thephotodiodes 826 b, so that the photodiode 826 b receives light filteredby the MRBR 824. In another embodiment, the MRBR 824 is not coated onthe photodiode 826 b, but is in the path of light 822 from the laser 810to the photodiode 826 b. The MRBR 824 preferably has a plurality ofnarrow reflectivity bands, preferably closely and substantially evenlyspaced, over the tuning range of a given laser 810 whose wavelength isto be monitored, with each discrete target wavelength of the laser 810corresponding to one of the MRBR 824 reflectivity bands' centerwavelength.

[0115] Each of the photodiodes 826 a, 826 b is monitored by acorresponding circuit 828 a, 828 b. The amount of the light 822transmitted through the HR mirror 816 that reaches the photodiodes 826a, 826 b corresponds to a change in electrical characteristics of thephotodiodes 826 a, 826 b. In one embodiment, the current flowing throughphotodiodes 826 a, 826 b changes in response to the amount of light 822received. The circuits 828 a, 828 b detect the change in electricalcharacteristics and report that change to a processor 830. In onembodiment, a single circuit performs the functions of 828 a, 828 b, and830.

[0116] In an embodiment, there is a one-to-one correspondence betweenthe MRBR 824 reflectivity bands and the desired selectable (tunable)wavelengths of the laser 810. In other embodiments, the MRBR 824 canhave other reflectivity bands between the target wavelengths, so long asthe locking algorithm is sophisticated enough to take the “extra”reflectivity bands into account when locking and changing wavelengths.

[0117] Monitoring photodiode 826 a is of the type typically used tomonitor for power, in a feedback loop which controls current poweringthe laser 810 (to maintain a constant output power). However, lightreaching 826 b is first filtered by passing through the MRBR 824, whichis coated onto the surface of the photodiode 826 b. The reflectivitybands of the MRBR 824 may have approximately 99% reflectivity, at thecenter wavelength thereof, and 97% or less reflectivity between thebands (troughs). The HR mirror 816 may have, for example, 99%reflectivity. Thus, the HR mirror 816 transmits out the “back side” ofthe laser 810 about 1% of the light lasing in the cavity 812. This 1%light 822 impinges on both photodiode 826 a and the MRBR 824. Of thelight reaching the coated photodiode 826 b, the light is either 99%reflected by a band reflectance, or reflected to a lesser degree if thelaser 810 is off band. In another embodiment (e.g., in a preferredembodiment for the embodiment described with reference to FIG. 5A), thephotodiode 826 b is not coated, but the path of light 822 is through theMRBR 824. If the laser 810 is off band, more light reaches photodiode826 b, than is the case when the laser 810 is on band (locked) becausethe reflectance of the MRBR 824 decreases as the wavelength varies fromthe on band frequency for small amounts of variance.

[0118] Thus, when the laser 810 is properly tuned to emit at one of thedesired wavelengths (e.g., one of the ITU wavelengths), only about 1% ofthe light 822 impinging on the MRBR 824 will be transmitted and reachphotodiode 826 b. If the wavelength starts to drift out of thereflectivity band, a larger fraction of the light 822 will be passedthrough the reflector/filter 824. The increase in light detected byphotodiode 826 b can permit the monitoring circuit/algorithm todetermine that wavelength drift has occurred, and to adjust the tuningparameter (e.g., temperature, gain, and/or current) to get back intowavelength lock.

[0119] As will be appreciated, non-coated (or unblocked) photodiode 826a can be used as a reference photodiode to monitor power, and to providea reference for photodiode 826 b. Photodiode 826 a is preferablydisposed next to photodiode 826 b, where possible, e.g. when anedge-emitting laser 810 is used, because the edge-emitting beam 822disperses widely. It is not necessary, though, for photodiode 826 a tobe disposed next to photodiode 826 b. The reference photodiode 826 a canbe used as a reference and to maintain power across channels (powerequalizer). Photodiode 826 b, normalized to photodiode 826 a, can beused by suitable circuitry or algorithm, see for example FIGS. 14-15,programmed into a digital signal processor (DSP) 830 to determine whenwavelength lock is being lost. This feedback can be used to tune thelaser 810 so as to regain wavelength lock. For example, a temperaturecontroller may be used to change the temperature of the laser interior818, to adjust the lasing wavelength. In an embodiment, the temperaturecontroller is located in the laser interior 818. In another embodiment,the temperature controller is proximate to the laser 810.

[0120] In an embodiment, a sufficiently sophisticated algorithm (e.g.,an algorithm run by a microprocessor or DSP 830 that controls currentinjected into the gain region of the laser 810) may be employed todistinguish power decreases due to aging from changes in light detecteddue to wavelength drift. For example, if the laser 810 is at a properwavelength (on band) then most of the light 822 is reflected by themirror because it is in the reflectivity band. However, if thewavelength drifts, the light detected by photodiode 826 b will increase.If the laser 810 loses power, however, due to aging, then the photodiode826 b will detect a gradual decrease in light. By contrast, loss ofwavelength lock increases the light detected by photodiode 826 b becauseless is reflected by the MRBR 824. Thus, if the wavelength is lockedinto a given channel, a measured increase in light means wavelengthdrift, and a decrease in light means aging-related power decreases. Inan embodiment, this technique may be used even in the absence ofphotodiode 826 a. In embodiments employing a separate, power-monitoringreference photodiode 826 a, however, the algorithm can take this intoaccount, by normalizing the measurement of photodiode 826 b to those ofphotodiode 826 a.

[0121] In alternative embodiments, other techniques, e.g. currentinjection or other techniques for changing the gain (and thus thewavelength), may be employed to tune the laser's wavelength. In a VECSEL(see FIGS. 10A-B) the effective length of the cavity can be modifiedresulting in a change in the wavelength of the laser output.

[0122] In an embodiment, as described above, the MRBR 824 is physicallyseparated from the HR mirror 816. It may be stand-alone or part of (acoating on) the photodiode 826 b. In an embodiment, the MRBR 824 may betilted at a slight angle (“off angle”) to minimize reflection(interference) back into the laser cavity 812.

[0123] In an alternative embodiment, only photodiode 826 b is employed;photodiode 826 a is absent. However, this approach may make it morecomplicated or difficult to monitor power independently of monitoringfor wavelength lock. In another embodiment, for example where onlyphotodiode 826 b is employed, the MRBR 824 is not coated on thephotodiode 826 b, but is instead part of the HR mirror 816 (or coatedthereon). Alternatively, other tapping techniques may be employed, e.g.a tap coupler 910, as illustrated in FIG. 9. In the system shown in FIG.9, the wavelength locker 916 contains the photodiodes 826 a, 826 b, anMRBR coating photodiode 826 b, and suitable circuitry to providefeedback for wavelength locking to the temperature controller 918.

[0124] In some applications, for higher frequency or more closely spacedtunable wavelengths, the reflectivity bands of the MRBR 824 need to beextremely close together. One way to achieve this is to design theappropriate MRBR layer structure (typically, a larger number of layers)to provide more closely-spaced reflectivity bands. However, it may beundesirable or impossible to fabricate a thick or complex enough layerstack structure to achieve very close reflectivity band spacing. Analternative technique is to combine an MRBR 824 having a smaller numberof bands, in combination with an air-gap/etalon effect with the HRmirror 816. That is, MRBR 824 combines with HR mirror 816 and theair-gap therebetween, to form an etalon reflector which itself has aseries of reflectivity bands. In this embodiment, the MRBR 824 isparallel to the HR mirror 816 and is separated by a small air distancesufficient to narrow the band spacing. Alternatively, instead of MRBR824, a standard DBR may be employed instead, to combine with HR mirror816 and an air gap to provide an etalon reflector with the desiredspaced reflectivity bands.

[0125] In other embodiments, VCSELs (including VECSELs) may be employedinstead of edge-emitting lasers. In such embodiments, the MRBR 824 maybe used to filter tapped laser light before it reaches photodiode 826 b.

[0126] In alternative embodiments (as described in further detailbelow), the MRBR may serve as one of the two cavity mirrors of a VCSEL(i.e., either the top or bottom mirror), or, it may serve as theexternal mirror of a VECSEL. In these cases, it can be more importantthat the reflectivity bands have a substantially constant reflectivityprofile, and that is also above the lasing threshold reflectivity. Thisis because power should not change too much from selected wavelength towavelength. It may be more expensive and/or difficult to fabricatesufficiently constant reflectivity band profile MRBRs, than to fabricateMRBRs with more variation in reflectivity from band to band. However,the latter type MRBR, with more unevenness in the band reflectivity fromwavelength to wavelength, which may be cheaper and easier to fabricate,may be sufficient for use as an external wavelength monitor/locker, asshown in FIGS. 8 or 9.

[0127] When the MRBR is not used as one of the cavity mirrors of aVCSEL, it is more important that the reflectivity bands be sufficientlynarrow and precisely spaced, than that they have uniform reflectivityfrom band to band, or even that the reflectivity of the bands be abovethe lasing threshold reflectivity mentioned above. So long as there is areflectivity band covering each target wavelength, locking photodiode826 b will detect an increase in detected intensity when lock starts tobe lost. In particular, photodiode 826 b will see a change from whateverthe detected intensity is while in lock, to a higher amount (forwavelength shift) or lower amount (for aging-cased decrease in power).This happens even if there is an uneven reflectivity peak profile.Alternatively, the rate of change may be measured; and/or thereflectivity peak profile may be known and taken into account by asuitable algorithm.

[0128] In an embodiment, a coarse tuning parameter (e.g. temperature,current) is used to adjust the wavelength of the laser. By monitoringthe lasing wavelength with the MRBR of the present invention, a suitablealgorithm can be employed to lock onto any of the center wavelengths ofthe target reflectivity band. For example, a given MRBR may havereflectivity bands centered at several substantially evenly-spacedwavelengths, λ₁, λ₂, . . . λ_(N). Initial calibration of a given lasercan be done to correlate a given tuning parameter with a givenwavelength window associated with one of the MRBR's reflectivity bandwavelengths. Where an MRBR as shown in FIGS. 1-4 is employed, with asubstantially unchanging reflectivity band profile, the coarse tuningparameter is expected to achieve a wavelength near the center wavelengthof the corresponding reflectivity band. The tuning parameter can bevaried under the control of a suitable algorithm, to reach the centerwavelength of the reflectivity band (e.g., by minimizing the lightdetected by photodiode 826 b, because maximum reflectivity, and thusminimum transmission, occurs when the laser is lasing at the centerwavelength of a given reflectivity band as described in FIG. 14). Onceit is known, e.g. by calibration, that the laser is locked onto a givenwavelength, the coarse tuning parameter(s) can be varied to quicklyselect another wavelength corresponding to the center wavelength ofanother reflectivity band, see FIG. 15.

[0129] Additionally, because the difference in light measured will besubstantially greater in the troughs than at the peaks, simple integercounting can be used to know exactly which channel the laser isoperating at, and, e.g., to quickly change channels (e.g., from channel2 to channel 7, by counting 5 pulses). I.e., by use of a suitablealgorithm, transitions over reflectivity bands can be counted, toprecisely select another discrete wavelength. Such counting is shown inFIG. 15.

[0130] As with the embodiments described above with reference to FIGS.8-9, the MRBR may be either part of the laser 810 or part of themonitoring module, e.g. coated on photodiode 826 b, between photodiode826 b and the HR mirror 816, or it may form the HR mirror 816 itself(assuming, for the last embodiment, that it has sufficient reflectivityto permit lasing to occur).

[0131] Although an MRBR may be employed having a substantially constantreflectivity band profile for wavelength monitoring (whether or not thereflectivity bands have a reflectivity above or below a lasing thresholdreflectivity), an MRBR may be employed which has a substantially varyingreflectivity profile (i.e., a varying peak profile and/or troughprofile) over the wavelength range of interest (e.g., tuning range). Forexample, an MRBR may be employed having a plurality of reflectivitypeaks which have a monotonically varying reflectivity band peak profileover the tuning range. Alternatively, the trough minima may be utilizedfor monitoring/locking, where the trough minima have a known, varyingtrough profile. In the former case, each reflectivity peak has a uniquereflectivity with respect to its neighbors, thereby establishing aunique reference for each discrete wavelength and thus permittingdetermination of the current lasing wavelength without counting.

[0132] The MRBR of FIGS. 5A-B, for example, may be employed as the MRBRof the laser system of FIG. 8, or of a similar system not havingphotodiode 826 a. In this embodiment, the MRBR has a monotonicallyvarying amplitude profile over the tuning range of the laser. The MRBRhas both reflectivity peaks and troughs (corresponding to transmissivitytroughs and peaks, respectively). Either or both the reflectivity peaksand trough minima may be used to lock onto a selected wavelength, andalso to determine which wavelength the laser is currently locked onto.In an embodiment, due to the unique reflectivity of each (or adjacent)reflectivity bands in the tuning range, the current lasing wavelengthcan be determined without counting. FIG. 14 shows this method of lockingonto a wavelength of either a peak or a trough minima.

[0133] In alternative embodiments, an MRBR 824 may be employed which hasa varying reflectivity trough profile as well, or instead of, a varyingpeak profile. For example, the MRBRs of FIGS. 2-4 have varying troughprofiles. In this case, the reflectivity trough minima (e.g.,transmissivity peak) may be used to achieve lock. The reflectivitytrough minima may be much narrower than are the reflectivity band peaks,and thus may provide more precise wavelength monitoring and locking.

[0134] In further embodiments, the peak (or trough) profile need notvary monotonically. As long as each neighboring (adjacent) peak (ortrough minima) varies from its neighbors in a known way, and to a greatenough degree to permit differences in measured intensity to beadequately distinguished, an appropriate locking algorithm can determinethe current lasing wavelength, without counting in some embodiments, andprovide for wavelength locking as described hereinabove.

[0135] Thus, in some embodiments, an MRBR is employed which has avarying reflectivity profile (i.e., a varying peak profile and/or troughprofile), in which neighboring reflectivity peaks (or trough minima,depending on which are used for locking) vary from each other inreflectivity substantially enough, and in a known way, so that anappropriate locking algorithm can determine the current lasingwavelength without counting. That is, the reflectivity profile is anon-constant function that varies sufficiently to permit the differencesin reflectivity of the peaks (or trough minima) to provide usefulinformation to the locking algorithm. The reflectivity profile may varymonotonically, for example substantially linearly, over thetuning/monitoring range, as in the MRBR of FIGS. 5A-B. As noted above,this MRBR has over a wavelength range of 1540 to 1580 nm, reflectivityband peaks spaced substantially uniformly about 1 nm apart whichmonotonically and substantially linearly decrease in reflectivity fromabout 70% to less than 10%. In particular, as illustrated in FIG. 5A,this MRBR has at least at least fourteen reflectivity (wavelength) bandsover a certain range, where the reflectivity peaks vary in reflectivityby more than 20%, but each deviates by less than 5% reflectivity from alinear relationship of length of wavelength to percentage ofreflectance.

[0136] In other embodiments, other MRBRs having substantially varyingreflectivity profiles may be employed. For example, other structures maybe employed for the MRBR to give rise to a substantially linearreflectivity profile in which each reflectivity peak within the tuningrange varies in reflectivity by at least a minimum threshold amount(e.g., 5% or 10%) from its neighboring peaks, and deviates by less thana maximum tolerance (e.g., 5%) from a linear relationship of wavelengthto percentage of reflectance. In other embodiments, the reflectivityprofile may be monotonic but non-linear, where the peaks deviate fromthis function by less than some maximum tolerance. Or, the reflectivityprofile function may be non-monotonic, such as the shape of the troughprofile of the MRBR shown in FIG. 3A.

[0137] In an embodiment, the reflectivity peaks each correspond to oneof the target lasing wavelengths of laser 810. In one embodiment, thepeak of each reflectivity band (i.e., its center wavelength) correspondsto the target wavelength. In this embodiment, if an MRBR having avarying reflectivity band profile is employed, then each selectablewavelength corresponds to a unique reflectivity (peak or troughminimum), and thus to a unique detected intensity upon lock (normalizingfor age- or power-related changes in intensity). Thus, a lockingalgorithm may be employed, which can keep the laser locked onto thedesired wavelength (by adjusting the laser's tuning parameter to preventwavelength drift), and which can also determine which wavelength thelaser is currently locked onto, without the necessity of the countingdescribed above for MRBRs having substantially constant reflectivityband profiles.

[0138] Wavelength drift in either direction will result in an increasein light detected by photodiode 826 b. Therefore, a sufficientlysophisticated locking algorithm is preferably employed, which candetermine in which direction the drift is occurring and correct it. Forexample, historical data or other data may be consulted to make a bestguess as to which direction the wavelength is drifting, in cases ofincreased light detection by photodiode 826 b. Or, an arbitrary guessmay be made. In either case, the algorithm adjusts the laser wavelength,hopefully in the appropriate direction, through the use of negativefeedback, until lock is regained. If the adjustment exacerbateswavelength drift, it can be assumed that the wrong assumption was madeand corrections in the appropriate direction can be made. See FIG. 14.

[0139] In another embodiment, a specified reflectivity point on the“side” of the band, less than the maximum reflectivity, is selected tocorrespond to the target wavelength. In such an embodiment, thedirection of wavelength drift is unambiguous. In this case, the lockingalgorithm constantly controls the tuning parameter to maintain thespecified point on the corresponding reflectivity band. In anembodiment, a reference photodiode 826 a may be used to correct forchanges in intensity caused by factors other than wavelength drift, e.g.aging. Another advantage of this approach is that it is not as criticalto design and fabricate an MRBR 824 where each band has its centerwavelength precisely matched with the target wavelengths. Instead, it issufficient that there be at least one reflectivity band (or side) pertarget wavelength, with a known profile, so that the exact targetwavelength can be correlated with a given percentage reflectivity of thepeak reflectivity of the corresponding band. For example, this isindicated by the exemplary solid dots on the sides of some of thereflectivity bands of FIG. 5A. (In an alternative embodiment, multipletarget wavelengths can correspond to different reflectivities on thesame side of a given reflectivity band.) In such an embodiment, themonitoring algorithm is still able to quickly determine the currentlasing wavelength without counting, by finding the peak of the currentreflectivity band (e.g., during an initialization period) andcorrelating its reflectivity (or its proxy, the lasing intensitymeasured by photodiode 826 b) with the appropriate band. Thereafter, thealgorithm can back off of the center of the band to find the appropriateplace on its side that corresponds with the target wavelength desired.

[0140] The varying-profile MRBR 824 may be employed either inconjunction with power-monitoring photodiode 826 a, or in an embodimentin which only photodiode 826 b is employed. Preferably, photodiode 826 ais used to stabilize power, and photodiode 826 b (normalized tophotodiode 826 a) is used to detect wavelength.

[0141] VECSEL Employing Single Lasing-Reflectivity Peak Reflector

[0142] The SRBR of FIG. 7 is a distributed (multilayer stack) reflectorwith a structure that gives rise to a single, narrow, high-reflectivitypeak or band, somewhere within a given wavelength range of interest, asdescribed above. In an embodiment, the single reflectivity band is anarrow lasing threshold reflectivity band (having reflectivity above alasing threshold reflectivity of, e.g., 99%) covering a singlewavelength of an ITU grid set of wavelengths (e.g., one of those around1.55 μm). I.e., Δλ₉₉ for the SRBR covers one of the ITU gridwavelengths. The high reflectivity figure corresponds to the lasingthreshold reflectivity for a VECSEL (e.g., typically 99% to 99.5%), ator near the gain spectrum maximum. Outside the single high-reflectivityband all other reflectivities of the SRBR in the wavelength range ofinterest are less than some second reflectivity threshold below thelasing threshold reflectivity, e.g. less than 97%.

[0143] For a given set of parameters, Δλ₉₉ encompasses a singlewavelength of the ITU grid, but the parameters can be changed to adjustΔλ₉₉ to cover another ITU grid wavelength. For example, for a givendistributed dielectric multilayer stack structure, changing therefractive index of one or more layers of the multilayer stack can shiftthe wavelength peak, i.e. the Δ80 ₉₉. This may be done by changing thetemperature of the SRBR, although this can be a slower way to change thereflectance spectrum of the SRBR than other techniques, such aspiezoelectric techniques. For example, some of the layers of an SRBR canconsist of piezoelectric or electrooptic material, in which case avoltage source can be used to provide a voltage (electric field) acrossthe piezoelectric layers of the SRBR to vary the refractive index, whichcan shift the reflectivity spectrum of the SRBR.

[0144] An SRBR may be utilized for a variety of applications, includinguse for wavelength locking onto a fixed wavelength or for use as thetuning element in a tunable VECSEL.

[0145] In an embodiment shown in FIG. 10A, an SRBR 1018 is used as thethird reflector of a VECSEL 1010. Its reflectivity spectrum may bechanged by changing its refractive index (or of some of its layers), forexample to provide a tunable laser system. Its refractive index may bechanged by, for example, wavelength tuning device 1030. In anembodiment, wavelength tuning device 1030 may be an optical pump laser,a voltage source, or a heating source. An optical pump laser, forexample, may work well if the SRBR has semiconductor layers for some orall of its layers, instead of only dielectric layers. However, it may bedifficult, impractical, or otherwise undesirable to fabricate an SRBRusing semiconductor layers. Thus, in an embodiment, device 1030 providesa means for changing other parameters of SRBR 1018 such as temperatureor voltage. For example, some of the layers of SRBR 1018 can consist ofpiezoelectric or electrooptic material, in which case device 1030 is avoltage source which provides a voltage (electric field) across thepiezoelectric layers of the SRBR to vary the refractive index, which canshift the reflectivity spectrum, and thus the wavelength covered by thesingle reflectivity peak, of the SRBR.

[0146] As shown in FIG. 10, a VECSEL 1010 has a two-section cavity 1012,with bottom mirror 1014, exit mirror 1016, and third mirror 1018 formedby the SRBR. The output 1021 of the VECSEL 1010 is through the exitmirror 1021. The presence of output at both angles of the two-sectionexternal cavity can decrease efficiency compared to the implementationof FIG. 10B. The VECSEL 1010 comprises an active region 1020, and apower source for electrically or optically pumping the active region.For an OP laser, the power source may be an external pumping laser (notshown); for an EP laser, the power source may be a current and/orvoltage source. In either case, the power source is functionally coupledto the active region when the pumping energy may be applied thereto bythe power source.

[0147] A given laser, such as VECSEL 1010, has a certain gain spectrum.The gain spectrum typically has a maximum at a particular wavelength.The gain spectrum, including its maximum, can be shifted with respect towavelength, e.g. by changing the temperature or pumping energy. Also,because the mirrors are not 100% reflective, a loss is introduced.Lasing is possible where there is sufficient reflectivity (i.e., wherethe gain exceeds the loss, i.e. where there is a net gain) and wherethere is a cavity mode. Typically a broad reflectivity spectrum isprovided by DBR cavity mirrors, so that there is a small loss over awide wavelength range which usually includes the gain spectrum maximum.There can be several cavity modes within the wavelength range wherelasing is possible, i.e. where there is net gain. Typically, the modeclosest to the net gain maximum will be selected and will win outthrough mode competition, although multi-mode operation and mode hoppingcan occur if too many cavity modes exist close together, close to thenet gain maximum.

[0148] In the present invention, the SRBR provides a narrow reflectivityband above the lasing threshold, so that the laser has net loss (lossexceeds gain) for all but a narrow lasing threshold reflectivity bandcovered by the peak of the single reflectivity band. Thus there is asmaller wavelength range over which lasing is possible, namely thenarrow lasing threshold reflectivity band encompassing the single peakof the SRBR, assuming it intersects the gain spectrum at a high enoughgain (e.g., at or near the gain maximum) so that the net gain is highenough to permit lasing to occur. In addition, lasing can only occur ifthe narrow lasing wavelength band of the single peak includes a cavitymode. The VECSEL 1010 will provide single-mode lasing at a cavity modewavelength within the single lasing threshold reflectivity band of theSRBR 1018 if it intersects the gain spectrum at a high enough gain.Because the lasing threshold reflectivity band is very narrow, therewill likely be only one cavity mode in the lasing threshold reflectivityband. Also, because of the narrowness of the lasing thresholdreflectivity band, the lasing wavelength will be approximately thecenter or peak wavelength of the single lasing threshold reflectivityband.

[0149] As an example, if the single lasing threshold reflectivity bandof SRBR 1018 covers an ITU grid wavelength and a cavity mode wavelength,and if the lasing threshold reflectivity band intersects the gainspectrum near the gain maximum, there can be a large net gain and stablelasing at the cavity mode wavelength, which is approximately the desiredITU grid wavelength. For comparison purposes, if the third mirror 1018instead had an MRBR with many closely-spaced reflectivity bands, changesin the gain spectrum could select one or more of the reflectivity bandsfor lasing; and mode hopping could occur in some applications. However,in an embodiment illustrated in FIG. 10A, third mirror 1018 comprises anSRBR, which, because of the single peak, can have superior mode-hoppingrejection characteristics. Also, the gain can be adjusted withoutaffecting the lasing wavelength, because lasing is only possible at thesingle peak wavelength.

[0150] Thus, by utilizing an SRBR as one of the mirrors in thethree-mirror, two-section cavity of a VECSEL, the reflectivity peakwavelength of the SRBR helps the laser lock onto that wavelength, andthe gain of the active region may be adjusted to some degreeindependently of the lasing wavelength.

[0151] The reflectivity peak of the SRBR can either be fixed or it canbe tunable. In the latter case, an MRBR or other type of filter may beemployed along with a wavelength locking circuit to tune the SRBR peakto the desired wavelength. Power monitoring elements and circuitry canbe used to adjust the gain independently.

[0152] It is often possible to independently tune or adjust gain and thecavity modes. For example, changing the pumping power shifts the gainspectrum, but only has relatively minor, secondary effects on the cavitymodes. By contrast, more changing the index of refraction of variouslayers of the cavity can shift the cavity mode. This may be done byadjusting the temperature of the laser. Thus, for example, assume aVECSEL or VCSEL employing an SRBR, which has a fixed wavelength singlepeak at the desired ITU grid lasing wavelength. When first using orcalibrating the laser, the pumping power and temperature are set atinitial levels. If there is no lasing, it can be assumed that the singlereflectivity peak does not intersect a cavity mode, i.e. the cavity modeis slightly “off”. Thus, the temperature can be adjusted by a controllerunder control of a suitable algorithm, to adjust the cavity modes, untillasing occurs, at which point the cavity mode has been calibrated to besufficiently close to the single reflectivity band and the desiredlasing wavelength. Preferably, the temperature (and thus wavelength ofthe cavity mode) is adjusted until the output power is maximized; atthis point, the cavity mode is precisely located at the peak of thesingle reflectivity band. Then, if the power needs to be higher orlower, the pumping power can be adjusted to adjust the gain spectrum. Ifonly minor power adjustments are needed, the cavity mode may not changeappreciably. However, if adjusting the gain spectrum to change theoutput power results in an appreciable shift in the cavity mode, thetemperature can again be adjusted to counter the second-ordercavity-mode shifting effect of the gain adjustment.

[0153] In an embodiment, SRBR is tunable to provide a tunable VECSEL. Inthis case, as illustrated in FIGS. 10A-B, a monitor photodiode 1024 andaccompanying measurement circuit 1026 measure the amount of lighttransmitted through the SRBR 1018. The light 1022 may be passed throughan MRBR or other wavelength-sensitive filter first, as described withreference to the wavelength locking function of FIG. 8. That measurementis then used by a processor 1028 to calculate a change in operation of areflectivity wavelength tuning device 1030 to obtain a particular peakwavelength for SRBR 1018. As discussed with respect to FIG. 8, monitorphotodiode 1024 can include an MRBR coating to assist in segregatingparticular desired wavelengths, or an MRBR or other type of wavelengthfilter placed between reflector 1018 and the monitor photodiode 1024.The same types of feedback algorithms discussed with respect to FIG. 8can also be used to modify SRBR 1018 in response to measurements of themonitor photodiode 1018.

[0154] Monitor photodiode 1024 can also be used independent ofwavelength to provide feedback indicative of the laser output power.That feedback can then be used to determine in part the proper pumpinglevel provided by the power source coupled to the active region 1020 asdisclosed with respect to FIG. 8. Alternatively, other taps or monitorphotodiodes may be employed to monitor the output intensity to adjustthe gain, independent of the lasing wavelength.

[0155] Thus, in an embodiment, as shown in FIG. 10A, the effectivewavelength of the single peak of the SRBR 1018 is changed, to tune thelasing wavelength of the VECSEL 1010. In an embodiment, the wavelengthtuning device 1030 is used to shift the single lasing thresholdreflectivity band of SRBR 1018 (and its peak reflectivity wavelength) upor down in wavelength, thereby adjusting the overall lasing wavelengthof the VECSEL 1010. As noted, device 1030 may use any suitable techniqueto change the reflectivity profile of the SRBR 1018, such as temperatureor piezoelectric techniques.

[0156] In another embodiment, the monitor photodiode 1024 is employed,e.g. with a tap as shown in FIG. 9, to monitor the actual lasingwavelength, and to provide feedback to the optical pump 1030 or otheradjustment device, to lock onto a desired wavelength. In thisembodiment, an external MRBR may be employed with the monitor photodiode1024, as described above, for the wavelength monitoring and lockingfunction. In yet another embodiment, a separate monitor may be employedto monitor the power of laser 1030, to ensure that it is supplying thedesired optical pumping to the SRBR 1018, which is calculated to achievea desired change in wavelength, under certain conditions. For example,the SRBR 1018 may be temperature controlled so that varying the opticalpump power in a known way changes the wavelength of the single peak in aknown way.

[0157] The various embodiments discussed above can also be implementedwith the cavity 1012 arranged around the SRBR 1018 with the exit mirror1016, and thus the output 1021, at one end of the cavity 1012, as shownin FIG. 10B.

[0158] In an alternative embodiment of the implementation illustrated inFIG. 10B, mirror 1018 is an SRBR and the exit mirror is an MRBR so thatthe laser only produces desired wavelengths and the particularwavelength produced depends upon the parameters (e.g., optical pumping,temperature, or voltage) applied to the SRBR.

[0159] In an alternative embodiment, an appropriately designed MRBR maybe used as an SRBR. For example, an MRBR that has sufficientlyspread-out reflectivity peaks can function as an SRBR if only one of itsreflectivity peaks lies within the wavelength range of interest, i.e.those wavelengths at which lasing might be desired. Such a reflector maybe referred to as an MRBR-type SRBR, or MRBR-SRBR. For example, thewavelength range of interest may be a set or subset of ITU gridwavelengths for which the gain spectrum of the active region is designedto operate; or it may be the wavelength range which can be covered by asufficiently high gain, e.g. within the range of wavelengths covered bythe FWHM of the gain spectrum, including wavelengths covered when thegain spectrum is shifted by adjusting its power source. In such a case,within the wavelength range of interest, the MRBR provides only a singlepeak; all other reflectivity peaks of the MRBR fall outside thewavelength range of interest. For example, the MRBR has a single peak inthe wavelength range of interest, which intersects the gain spectrum ata sufficiently high gain so that there is net gain; and all other peaksintersect the gain spectrum at a much lower gain so that there is verylow, or negative, net gain, in which case there will be single-modelasing at only the single peak's wavelength. Thus, for such anSRBR/MRBR, within the gain spectrum range of interest, there is only asingle reflectivity peak; and all other reflectivities outside thesingle peak or band, in the wavelength range of interest, are below thisthreshold. That is, within the wavelength range of interest (i.e., thewavelength range covered by the gain spectrum range), there is only asingle reflectivity peak of the MRBR that is above the lasing threshold;all other reflectivities of the MRBR in the spectrum of interest areless than some second threshold below the first one, e.g. less than 97%.

[0160] As described below, in alternative embodiments an SRBR may beemployed for wavelength locking with an integrated, monolithic VCSEL,and with a one-section VECSEL.

[0161] Wavelength-locked Laser with Multiple Reflectivity Band Reflector

[0162] In an embodiment, similar to the configuration shown in FIG. 10Aor 10B (where the third mirror 1018 is an MRBR instead of an SRBR), aVECSEL 1010 employs a bottom mirror 1014, an exit mirror 1016, and anexternal “top” reflector 1018 to complete the two-section cavity 1012.One of the three mirrors of the VECSEL 1010, preferably the external,“top” reflector 1018, is an MRBR in accordance with the presentinvention, having reflectivity bands which are spaced along targetselectable lasing wavelengths (preferably along the ITU grid). TheVECSEL structure, including cavity length and phase-matching layers, ispreferably also designed so that cavity modes fall along each of thetarget selectable lasing wavelengths.

[0163] If the MRBR is the third mirror 1018, the reflectivities of thereflectivity bands need to be above the lasing threshold reflectivity,e.g. 99.5%. This permits sufficient reflection in the lasing cavity 1012at one of the desired wavelengths. For example, the MRBRs of FIGS. 1-4can be used for this purpose.

[0164] The active region 1020 of the VECSEL 1010 can be eitherelectrically or optically pumped. By adjusting the electrical or opticalpumping power, the gain spectrum of the VECSEL 1010 can be shifted. Asthe gain spectrum shifts, it overlaps with different reflectivity bandsof the MRBR, causing discrete “jumping” from one lasing wavelength toanother. The bands are preferably far enough apart (e.g., a CWDM typespacing) to prevent mode hopping and to achieve single mode operation.Thus, as long as the coarse gain spectrum tuning of the active region1020 selects close to the desired wavelength range covered by the targetreflectivity band, a stable lasing wavelength at the narrow wavelengthof the reflectivity band will be achieved. This can avoid or reduce theneed for feedback that is often required in continuously tunable lasers.In another embodiment, feedback can be implemented to fine tune to peaksof the MRBR 1018, adjusting the MRBR band positioning through opticalpumping, temperature control, or piezoelectric voltage control asdiscussed above.

[0165] In other alternative embodiments, either bottom mirror 1014 orexit mirror 1016 may comprise an MRBR. If bottom mirror 1014 is theMRBR, then as for top mirror 1018, the reflectivity bands need to havereflectivity above the lasing threshold reflectivity. If exit mirror1016 is the MRBR, the reflectivity bands cannot have 100% or too highreflectivity (e.g., it should be somewhat smaller than the reflectivityof the bottom mirror 1014, which typically has a reflectivity >99.8%),because some light must be transmitted as output 1021.

[0166] In still further alternative embodiments, two of the mirrors1014,1018 of the VECSEL 1010 may have MRBRs, which can combine in a“vernier effect” to provide further wavelength selectability.

[0167] As described below, in alternative embodiments an MRBR may beemployed with an integrated, monolithic VCSEL, and with a one-sectionVECSEL.

[0168] Integrated Wavelength-locked VCSEL with Multiple or SingleReflectivity Band Reflector

[0169] In an embodiment, as illustrated in FIG. 11, an integrated VCSELemploys a bottom mirror 1112 and a top exit mirror 1116 to complete thelaser cavity. The structure is supported by a substrate 1110 with thelaser output 1118 perpendicular to the substrate 1110. One of the twomirrors of the VCSEL, preferably the bottom reflector 1112, is an MRBRin accordance with the present invention, having reflectivity bandswhich are spaced along target selectable lasing wavelengths (preferablyalong the ITU grid). The VCSEL operates similarly to thewavelength-locked VECSEL with MRBR as described above. The active region1114 of the VCSEL is preferably electrically pumped and tunable byelectrically changing its gain spectrum. In this embodiment, thereflectivity band profile of the MRBR (e.g. 1112) is fixed. Targetwavelengths are selected by changing the gain spectrum of the VECSEL,and/or by making any changes in the cavity modes by appropriatetemperature tuning. In an alternative embodiment, the MRBR forms exitmirror 1116, instead of bottom mirror 1112.

[0170] In another embodiment, an SRBR is used to form either exit mirror1116 or bottom mirror 1112. In this case, the lasing wavelength can befixed, while permitting gain to be adjusted. Alternatively, if tuning isdesired, bottom mirror 1112, for example, can be an SRBR, suitabletechniques (e.g., piezoelectric) may be employed to change thereflectivity profile of the SRBR, and thus to change the lasingwavelength. In such a case, for example, extra terminals may need to beprovided to permit piezoelectric tuning of the SRBR, independent fromproviding electrical pumping to the gain region 1114.

[0171] One-Section External Cavity Wavelength-locked VECSEL withMultiple or Single Reflectivity Band Reflector

[0172] In an alternative embodiment, there is provided a one-sectioncavity, non-integrated VECSEL, in which the top (exit) mirror isexternally mounted above an integrated bottom laser portion having theactive region and bottom mirror. One of the mirrors, either the top orbottom mirror, is either an SRBR or MRBR, to provide for wavelengthlocking on the wavelengths associated with reflectivity peak(s) of theSRBR or MRBR of the VECSEL. The VECSEL operates similarly to thewavelength-locked VECSEL with MRBR or SRBR as described above.

[0173] Reflectively Coupled Zigzag Waveguide Device for WavelengthLocking

[0174] As noted above, U.S. Pat. No.5,894,535 and Brian E. Lemoff etal., “A Compact, Low-Cost WDM Transceiver for the LAN,” 2000Proceedings, 50^(th) Electronic Components & Technology Conf. (IEEE2000) disclose a reflectively coupled zigzag waveguide device which isused for wavelength demultiplexing. The '535 patent, for example,discloses a device including a dielectric waveguide that guides a WDMsignal through a zigzag path. The WDM signal contains multiple lightsignals at several discrete wavelengths of light, e.g. λ₁, λ₂, . . .λ_(N). At particular vertices of the path optical filters selectivelytransmit and reflect wavelengths of light. Each of the filters has aunique passband centered on (having a transmissivity maximum at) one ofthe plurality of wavelengths λ₁, λ₂, . . . λ_(N). Each filter(relatively) passes light in the passband centered on its respectivewavelength λ_(i), and relatively reflects light outside this passband.Each filter may therefore be referred to as a mirror/filter. The zigzagwaveguide device of the '535 patent outputs, at each of the plurality ofmirror/filters, different wavelengths of light, λ₁, λ₂, . . . λ_(N),thus demultiplexing the WDM signal to extract individual signals at eachof the discrete wavelengths making up the WDM signal.

[0175] In the present invention, a reflectively coupled zigzag waveguidedevice is implemented as a wavelength locker instead of as ademultiplexer. Referring now to FIG. 16, there is shown a zigzagwaveguide device-based wavelength locking apparatus 1600, in accordancewith an embodiment of the present invention. Apparatus 1600 compriseszigzag waveguide device 1630, coupled to fiber 1610 to receive a tap oflasing light from an external tunable laser (not shown). Zigzagwaveguide device 1630 is a zigzag patterned dielectric channel waveguidestructure that guides a light signal through a zigzag path. Atparticular vertices of the path optical filters selectively transmit andreflect wavelengths of light. Zigzag waveguide device 1630 may also bereferred to as a reflectively coupled optical waveguide structure, whichis embedded in a substrate. Such a structure is a planar optical devicethat includes two or more optical channel waveguides oriented such thattwo adjacent waveguides converge at a vertex.

[0176] The tap of laser light received is at a current actual wavelengthλ_(A), which may or may not be substantially equal to the desired ortarget lasing wavelength λ_(L). The desired lasing wavelength λ_(L) is aparticular one of a plurality of discrete lasing wavelengths λ₁, λ₂, . .. λ_(N), i.e. L=1, 2, 3, . . . or N. If λ_(A)=λ_(L), laser lock has beenachieved. If not, the laser's lasing wavelength needs to be adjusted tomove λ_(A) in the direction of λ_(L), until they are equal orsufficiently equal (i.e., the difference λ_(A) and λ_(L) is less thansome predetermined threshold difference). The purpose of wavelengthlocking apparatus 1600 is to generate control signals to perform thisadjusting, so as to achieve and/or maintain laser lock on the desiredlasing wavelength λ_(L).

[0177] Zigzag waveguide device 1630 comprises a plurality N of opticalfilters/reflectors 1614, each filter 1614 _(i) having a passband at arespective wavelength λ_(i) which is a corresponding one of the Ndiscrete wavelengths λ₁, λ₂, . . . λ_(N). In an embodiment, the filters1614 are substantially identical except that their structures areadapted to provide different passbands. Filter 1614 may be distributeddielectric multilayer stack reflectors, for example.

[0178] As noted above, each filter 1614 _(i) (relatively) passes lightin the passband centered on its respective wavelength λ_(i), andrelatively reflects light outside this passband. Further, because thepassband is not ideal, light at exactly the center wavelength istransmitted at a higher degree than is light not exactly at the centerwavelength λ_(i) but still close to wavelength λ_(i) and within thepassband. Each filter 1614 _(i) therefore produces a respective filteredoutput signal, related to the intensity of light impinging thereon aswell as the wavelength of such light. Light within the passband producesa larger output signal the greater its intensity and also the closer itis to the center wavelength λ_(i). Apparatus 1600 also comprises acorresponding plurality N of photosensors, e.g. photodiodes, andmeasuring circuit 1618, arranged so that a photodiode and measuringcircuit 1618 _(i) is positioned to received the filtered output of thecorresponding filter 1614 _(i). In the illustrated embodiment, N=4.

[0179] The input optical signal at actual lasing wavelength λ_(A)propagates through a waveguide 1612 of zigzag device 1630 until itreaches a first filter 1614 a at a vertex of the zigzag patternedwaveguide structure 1612. In an embodiment, each vertex of the zigzagwaveguide structure 1612 has a vertex angle of approximately 12°, butmay be at other angles in other embodiments, e.g. within a range of 3°to 45°. Zigzag waveguide structure 1612 is formed by a number ofdielectric channel waveguides, comprised of dielectric layers embeddedin a cladding region 1619 of the substrate. The cladding region can be adielectric layer on the substrate or it can be the substrate itself. Ineither case the waveguides may be said to be embedded and/or patternedin a substrate. The cladding region 1619 has a cladding refractiveindex, and the dielectric channel waveguides of structure 1612 have awaveguide refractive index, which is higher than the cladding refractiveindex such that light is confined within the waveguide structure.Waveguide structure 1612 is embedded in the substrate using known meansand methods.

[0180] In an embodiment, each waveguide of the zigzag waveguidestructure 1612 is substantially rectangular in cross section and isapproximately 70 microns high by 100 microns wide. Other dimensions andshapes can also be employed for the waveguides.

[0181] The first optical filter 1614 a includes layers with the opticalproperty of transmitting a wavelength band (passband) centered on aparticular wavelength, e.g. λ₁, but reflecting other wavelengths atleast relative to the transmittance of wavelengths within the passband.Thus, the optical filter transmits light in a particular wavelengthrange (passband) out of the waveguide structure and reflects light inother wavelength ranges into the subsequent waveguide.

[0182] The reflected light continues to travel through the waveguideuntil it reaches a full spectrum reflector 1616. Substantially all thelight is reflected from the full spectrum reflector 1616 and continuesto the next vertex at which is located second reflector/filter 1614 b,which is similar to the first reflector 1614 a with a differentwavelength band, namely one centered on λ₂. The third reflector 1614 cand fourth reflector 1614 d are also configured to have highertransmittance for light in their corresponding wavelength bands centeredon wavelengths λ₃ and λ₄. The light transmitted by each of thereflectors 1614 a-d is detected by a corresponding photodiode andmeasuring circuit 1618 a-d.

[0183] Wavelength locking apparatus 1600 comprises a wavelength lockercircuit 1620, which may be, or comprise, a processor. Wavelength lockercircuit 1620 receives the signals generated by the N photodiode andmeasuring circuits 1618. That is, wavelength locker circuit 1620receives N signals, each signal corresponding to the intensity of lightwithin a respective passband. It may also receive the output from apower monitoring photodiode (not shown), for use as a reference, asdescribed below with reference to FIG. 12. Wavelength locker circuit1620 is presumed to have knowledge of the desired lasing wavelength,i.e. it knows which of the possible lasing wavelengths (λ₁, λ₂, . . .λ_(N)) is the desired wavelength λ_(L).

[0184] Wavelength locker circuit 1620 analyzes the signals with asuitable algorithm and controls the transmitting laser in accordancetherewith. In particular, based on the photodiodes' signals, wavelengthlocker circuit 1620 determines whether the laser is lasing at thedesired lasing wavelength λ_(L), that is, whether λ_(A)=λ_(L). If not,circuit 1620 generates control signals to provide to driver controlcircuitry coupled to the tunable laser, to adjust the lasing wavelengthof the laser so as to achieve lock on the desired lasing wavelengthλ_(L). If λ_(L) is λ₂, for example, then circuit 1620 makes sure thatthe output signal from photodiode and measuring circuit 1618 b is at amaximum.

[0185] As an example, if λ₂ is selected to be the desired lasingwavelength, then circuit 1620 (or some other circuit) sends controlsignals predetermined to achieve this lasing wavelength to the laser.This might not achieve the exact wavelength λ₂, however, due to slightmismatches in calibration or aging. For example, the laser may be lasingat actual wavelength λ_(A) which is close to, but not exactly equal to,λ₂. In this case, the outputs of photodiode and measuring circuits 1618a, 1618 c, and 1618 d (corresponding to wavelength passbands at λ₁, λ₃,and λ₄, respectively) will have zero or very low outputs, since actualwavelength λ_(A) is not equal or even close to any of these wavelengths,and thus nothing will be passed by reflectors/filters 1614 a, 1614 c,and 1614 d, respectively. However, λ_(A) will be still within thepassband of reflector/filter 1614 b, because it is close to λ₂. Becauseλ_(A) is not exactly equal to λ₂, less light will be passed throughfilter 1614 b than light at λ₂ would, because λ_(A) is not at the centerof the passband. In this case, circuit 1620 will be able to determinethat exact lock has not been achieved ,because the signal fromphotodiode and measuring circuit 1618 b will not be maximized and/orwill not be as high as a “lock threshold” (which may be determined withreference to a power monitoring photodiode signal). In another case, theinitial actual lasing wavelength λ_(A) may be closer or equal to thewrong discrete lasing wavelength λ₁, λ₃, or λ₄, than to targetwavelength at λ₂. In this case circuit 1620 can determine this by anappropriate algorithm to vary the lasing wavelength and analyzing theresulting signals output from all four photodiode and measuring circuits1618 a, 1618 b, 1618 c, and 1618 d. Other tuning and locking algorithmsare described below with reference to FIGS. 14 and 15. As will beappreciated, these and other wavelength locking algorithms can be usedto achieve, and also to maintain, wavelength lock.

[0186] Wavelength locker circuit 1620 can also take into accountexpected or predetermined losses occurring due to reflection losses atthe vertices. For example, there may be more loss to the signaltraveling in waveguide structure 1612 by the time it reaches the lastfilter 1614 d than when it impinged on the first filter 1614 a. Thus,circuit 1620 may take this into account when analyzing the signalreceived from each filter, i.e. it may employ a lower lock threshold foroutputs further down the waveguide structure 1612.

[0187] Thus, instead of extracting individual wavelength signals of theWDM signal for further transmission or routing, the zigzag waveguidedevice of the present invention produces a plurality N of signals basedon a (preferably single-wavelength) input signal having an unknownand/or potentially variable wavelength λ_(A), where each of the Nsignals is related in a known manner to the intensity of light in theinput signal at a particular discrete wavelength λ_(i) of a plurality Nof discrete wavelengths λ₁, λ₂, . . . λ_(N). In an embodiment of thepresent invention, a photosensor, such as a photodiode, is placed at theoutput of each filter, to monitor the intensity of light at thewavelength λ_(i) of the corresponding filter i. In this manner, as inthe wavelength monitoring and locking techniques described above, e.g.with reference to FIG. 8, wavelength locking may be achieved using azigzag waveguide device.

[0188] PLC Module for Conditioning Tunable Laser Output

[0189] Planar lightwave circuits (PLCs) are used as waveguides invarious optical applications. A PLC typically comprises a slab(substrate) of dielectric material (e.g., silica or silicon) into whichone or more waveguides are formed or “buried”. The waveguides areusually simple and well-defined waveguide structures, typically having asubstantially rectangular cross section. The waveguides serve asdielectric propagation media, for which the principle of operation isthe same as that for optical fibers that are circular in cross section.Various parameters such as width, thickness, and refractive indicesdetermine the operating wavelength and the modes a PLC waveguide willsupport. For example, the PLC may be designed to have single-modewaveguides. Also, waveguides may be tapered or shaped at their ends tomatch the mode profile of the fibers or other devices to which thewaveguide end is to be optically coupled.

[0190] PLCs using silica-based optical waveguides are fabricated on(embedded or patterned in) a silicon or silica substrate by varioustechniques, typically a combination of flame hydrolysis deposition (FHD)and reactive ion etching (RIE). First, fine glass particles are producedin an oxy-hydrogen flame and deposited on a host substrate (Si or SiO₂).After depositing the undercladding and core glass layers, the wafer isheated to a high temperature for consolidation. The circuit (waveguide)pattern is fabricated by means of photolithography and reactive ionetching (RIE). The core ridge structures are covered with anovercladding layer and consolidated again. Various kinds of waveguidesare in use, such as N×N star couplers, N×N arrayed-waveguide grating(AWG) multiplexers and other types of AWGs, optical add/dropmultiplexers (ADMs), and N×N matrix switches. Also, the silica-basedwaveguide on a Si substrate may be used as an optical hybrid integratingplatform, since Si has highly stable mechanical and thermal propertiesthat make it suitable as an optical bench. This permits optoelectronicdevices to be formed on the PLC using precise Si “terrace” alignmentfeatures, which helps to precisely align the device with respect to thewaveguides, to minimize optical coupling losses. Further information onPLCs may be found in Katsunari Okamoto, Fundamentals of OpticalWaveguides (Academic Press, 2000), chapter 9, “Planar LightwaveCircuits,” pp. 341-400 et pass.

[0191] A PLC module having a plurality of waveguides is used in anembodiment of the invention to condition a tunable laser output.Referring now to FIG. 12, in an embodiment 1200, a PLC module 1220 isused to condition the output of a semiconductor laser, preferably atunable laser, such as the VCSEL 1210 shown in FIG. 12. The output ofthe VCSEL 1210 may be coupled to the PLC 1220 by a fiber or othersuitable means. PLC module 1220 contains a plurality of patternedsilicon oxide waveguides 1221, 1222, 1223 patterned into a silica orsilicon slab via standard PLC fabrication techniques. In general, thewaveguides 1221, 1222, 1223 may be said to be embedded and/or patternedin a substrate. In embodiments in which the substrate is a silicon orsilica substrate, the substrate may be referred to as a silicon-basedsubstrate.

[0192] The plurality of waveguides include a primary or main waveguide1221 for coupling the laser output to some conditioning device 1224 suchas an SOA (semiconductor optical amplifier) and/or modulator (not shown)and then to a fiber 1225. The waveguides also comprise a plurality ofsecondary or “splitter” waveguides 1222, 1223 for transmitting smallportions of light tapped from the main waveguide 1221. The secondarywaveguides comprise first secondary or splitter waveguide 1223 andsecond secondary or splitter waveguide 1222.

[0193] Primary waveguide 1221 has an input end input receiving lightfrom the tunable laser 1210 and an output end for outputting said light,e.g., to a fiber 1225 for further transmission, preferably afterconditioning the light by some device in the waveguide, between itsinput and output ends, such as SOA 1224. Thus, the primary waveguidecomprises a conditioning device between its input and output ends forconditioning light received from the tunable laser so that the primarywaveguide provides, at its output end, light from the tunable laserconditioned by the conditioning device.

[0194] Secondary or splitter waveguides 1222 and 1223 provide a portionof light tapped from primary waveguide 1221 and apply it to othercomponents, such as a power monitoring photodiode 1226 and a filter1227. In an alternative embodiment, PLC 1220 comprises primary waveguide1221, secondary waveguide 1223, and filter 1227, but may not includesecondary waveguide 1222 and photodiode 1226.

[0195]FIG. 12 shows the functional relationship between the mainwaveguide 1221 and the splitter waveguides 1222, 1223. Severalstructures can be used to couple portions of the VCSEL output from themain waveguide 1221 to the splitter waveguides 1222, 1223. In anembodiment, the splitter waveguides use evanescent coupling to tap off aportion of light from the main waveguide 1221 or another of the splitterwaveguides which is itself directly or indirectly coupled to the mainwaveguide. In this embodiment, the waveguides are arranged so that aportion of each of the splitter waveguides is located in close proximityand parallel to a portion of the main waveguide. This proximity allowsfor evanescent waves corresponding to the VCSEL output in the mainwaveguide to propagate through the splitter waveguides. Thus, in thisembodiment, each secondary waveguide receives a respective portion oflight, from the primary waveguide, by either direct or indirectevanescent coupling from said primary waveguide. A secondary waveguidereceives its respective portion of light from the primary waveguide bydirect evanescent coupling if it is directly adjacent said primarywaveguide so that there is evanescent coupling between the two. If asecondary waveguide receives a portion of light from another secondarywaveguide (which is itself either directly or indirectly evanescentlycoupled to the main waveguide), by either evanescent or other coupling,it may be said to be indirectly evanescently coupled to the mainwaveguide.

[0196] In another embodiment, at least one of the splitter waveguidestaps off some light from the main waveguide 1221 by evanescent splittingand the next taps light from this last splitter waveguide, and so forth.For example, second splitter waveguide 1222 taps off a portion of lightfrom main waveguide 1221, and first splitter waveguide 1223 taps off aportion of the light from first splitter waveguide 1222. In anotherembodiment, instead of evanescent splitting an optical tap 1215 is usedto direct portions of the VCSEL output into the splitter waveguides.

[0197] In an embodiment, the second splitter waveguide 1222 applies thetapped light (a “first portion” of light from the main waveguide) to apower monitoring photodiode 1226 for power monitoring purposes, and thefirst splitter waveguide 1223 applies a second portion of tapped lightto a wavelength filter 1227 for wavelength locking purposes. A processor1230 employs an algorithm that responds to output from the filter 1227(and also photodiode 1226 in an embodiment) to generate control signalswhich are fed to driver circuitry 1232 by control line 1231. This may beany a suitable algorithm such as described above with reference to FIG.16, or other types of algorithms such as ones based on or similar toaspects of the algorithms of FIGS. 14-15. Control line 1231 in anembodiment carries digital signals instructing the driver circuitry howmuch to change the wavelength and drive current provided to tunableVCSEL 1210. In response to these digital control signals, drivercircuitry 1232 generates the appropriate analog voltage and/or currentsignals to effect the desired changes in tunable VCSEL 1210.

[0198] In an embodiment, filter 1227 provides at least a signal relatedto the intensity of light at the desired lasing wavelength λ_(L) whichis impinging on filter 1227 from first splitter waveguide 1223. Thus,for example, if tunable laser 1210 is designed to emit at one of fourselectable wavelengths λ₁, λ₂, λ₃, λ₄, and at a given time is supposedto be emitting at wavelength λ_(L)=λ₁, filter 1227 will output a signalcorresponding to the intensity of light at wavelength λ₁. If thisreading is at a maximum and/or above a certain threshold (determinedusing the output of power monitoring photodiode 1226 as a reference, inan embodiment) then there is laser lock; if not, then processor 1230sends an appropriate control signal to VCSEL 1210 to adjust its lasingwavelength, until the desired wavelength is achieved.

[0199] As noted above with respect to the discussion of FIG. 16, in anembodiment processor 1230 is supplied with information as to which ofthe possible target lasing wavelengths (λ₁, λ₂, λ₃, λ₄) is the currenttarget lasing wavelength λ_(L). In an embodiment the filter 1227 is atunable filter, in which any of wavelengths λ₁, λ₂, λ₃, λ₄ can beselected by an appropriate control signal to be the center wavelengthfor the filter's passband. In this case, filter 1227 is tuned to filterpass light at a passband centered on the target lasing wavelength λ_(L).It may do this in response to a control signal provided to it fromprocessor 1230, or other device, to select this passband.

[0200] In an alternative embodiment, filter 1227 is not tunable but is amultiple-output filter providing a plurality N of signals relatedrespectively to the intensity of light at each of a plurality ofdiscrete wavelengths λ₁, λ₂, . . . λ_(N) which impinge on filter 1227from first splitter waveguide 1223. In this embodiment, filter 1227 hasa plurality of dedicated filters, each having a passband correspondingto one of the N discrete wavelengths at which lasing may be desired.Thus, for example, if tunable laser 1210 is designed to emit at one offour selectable wavelengths λ₁, λ₂,λ₃, λ₄, and at a given time isactually emitting at wavelength λ₁, filter 1227 will output a maximumintensity reading for wavelength λ₁ and minimal readings for the otherwavelengths λ₂, λ₃, λ₄.

[0201] In one implementation of the multiple-output filter embodiment,the filter is formed from a reflectively coupled zigzag waveguide deviceas described above with respect to FIG. 16.

[0202] In another implementation of the multiple-output filterembodiment, the filter is formed from the filter shown in FIG. 13. Asshown in FIG. 13, the filter portion 1227 may comprise a plurality ofidentical filters 1314 a-d (instead of filters each having a differentwavelength, corresponding to λ₁, λ₂, . . . λ_(N), as in the zigzagwaveguide device 1630 of FIG. 16). These filters may be distributeddielectric multilayer stack reflectors, for example, having the desiredreflectance/transmissivity characteristic. The angle at which lightimpinges on such a filter affects the wavelength passband of the filter,for example because it affects the effective layer thickness seen by thelight. In the embodiment of filter 1227 illustrated in FIG. 13, filter1227 receives the portion of light via splitter waveguide 1223 andfurther splits this light into N=4 further secondary or splitterwaveguides 1312 a-d, via tap 1310. Tap 1310 may be similar to tap 1215of FIG. 12. Each of the splitter waveguides 1312 a-d are patterned toterminate at a unique angle Φ_(i) with respect to a filter 1314 a-d, sothat the identical filter 1314 a-d combined with the angle results in aspecified wavelength passband for the filter, for wavelength lockingpurposes. That is, the angle Φ_(i) for each filter 1314 _(i) is selectedso that the filter 1314 _(i), given its multilayer stack configuration,will yield the desired passband center wavelength λ_(i). In anembodiment, angle Φ_(i) may be regarded as the angle from normal to theparallel surface of the reflector 1314 _(i).

[0203] In particular, each filter 1314 a-d has an identical dielectricmultilayer stack configuration, which provides transmission passbandscentered at λ₁, λ₂, . . . λ_(N), when light impinges on the reflector atangles of Φ_(a), Φ_(b), Φ_(c), and Φ_(d), respectively. In theembodiment illustrated, for example, Φ_(a) (0°) <Φ_(b)<Φ_(c)<Φ_(d)<90°.In an alternative embodiment, all the angles, including Φ_(a), aregreater than 0° to avoid backreflection into the source. The lighttransmitted through the specified passband of each filter is detected byphotodiode and measuring circuit 1316 a-d, and the signals transmittedto processor 1230 of FIG. 12.

[0204] Wavelength Locking Algorithms for Tunable Lasers Employing anSRBR or MRBR

[0205] Algorithms are used to lock a light source onto the peak ortrough of a filter in accordance with an embodiment of the presentinvention. FIG. 14 depicts a flowchart of a method 1400 for locking ontoa peak or trough. If λ_(i) is the desired wavelength, a particulartuning parameter of the light source is initially specified to correlatewith the wavelength band of the filter (whether it is a SRBR or MRBR)that has a peak or trough at λ_(i). The tuning value could be, forexample, temperature of the laser or the degree of pumping whetherelectrical or optical. In an alternate embodiment, a separate wavelengthmeasurement method can be used to tune the light source to thewavelength band. Once the light source is producing a wavelength withinthe band, the method 1400 is different depending on whether λ_(i) is thepeak or trough of the band. (A peak of a band depicted by reflectance v.wavelength is a reflective maxima, while the trough of such an inverseband has a reflective minima.)

[0206] For a reflective minima, the output of the filter at the initialtuning parameter specification is measured and the tuning value is thenchanged by a small positive step. A new output is measured at thechanged tuning value. The new output is compared to the initial outputand the new output becomes the initial output if it is the greater. Onceone successful positive step has occurred and the new output has becomethe initial output at least once, the first unsuccessful positive stepthereafter indicates that the reflective minima has been over stepped byone and will result in the setting of the tuning value at the previousvalue. If no successful positive steps occur, i.e., new output is lessthan initial output for the first positive step, negative steps areattempted. The first negative step that is unsuccessful results in thesetting of the tuning value at the previous value. Thus, the algorithmtries both directions in the tuning value in an attempt to find thereflective minima and stops changing the tuning value once it has beenfound. The method for finding a reflective maxima is the same exceptthat the result of comparing the initial output and the new output isinverted. A new output that is less than the initial output isconsidered successful so that the new output replaces the initial outputand further steps in the same direction of the tuning value areattempted until a reflective maxima has been overstepped by one. In anembodiment, the method of FIG. 14 to reattain the reflective maxima orminima is periodically employed to correct any wavelength drift that hasoccurred since the last setting. In an alternative embodiment, otheralgorithms may be employed, e.g. a predictive algorithm may be employedwhen the current lasing wavelength is far away from the targetwavelength.

[0207] Algorithms are also used to move from one peak or trough minimaof a filter in accordance with an embodiment of the present invention toanother such peak or trough minima. FIG. 15 depicts a flowchart of amethod for moving a laser from a present wavelength, λ_(i), to a desiredwavelength, λ_(N). The method of FIG. 15 assumes that filter wavelengthsare defined by reflectivity maxima or peaks, though wavelengths definedby reflectivity minima or troughs could be changed by inverting thecomparison steps as in FIG. 14. As in FIG. 14, the tuning parameter canbe any laser parameter that corresponds to the wavelength of lightoutput by the laser, e.g., laser temperature or pumping level. Thealgorithm determines the number of bands between λ_(i) and λ_(N). Thetuning parameter is changed by a specified value and the output ismeasured. The output is then compared to a band threshold, which is someamount of reflection that is the minimum considered to be in a band. Ifthe output is still in the current band, the tuning parameter ismodified by the specified amount repeatedly until the output indicatesthat the laser wavelength has moved out of the band. Once the output isout of a band, the tuning parameter is then modified by the specifiedvalue until the output moves into the next band as indicated bycomparing the output with the band threshold. The number of bandstraversed, which starts at one, is incremented and compared with thenumber between that was initially determined. If more bands must betraversed to reach the band containing λ_(N), the process iterates. Ifthe band containing λ_(N) has been reached, then a one directionalimplementation of the method shown in FIG. 14 is used to find thereflective maxima corresponding to λ_(N). It is preferable that thesecond specified value be smaller than the specified value used forcounting bands, because fine tuning the reflective maxima requiressmaller steps than identifying and counting the bands between thedesired wavelength and the initial wavelength. In some embodiment, thevarious bands will have different reflectivities and comparison to thoseknown reflectivities can be used instead of band counting to locate thedesired wavelength.

[0208] In the embodiments described above, the MRBR and SRBR of thepresent invention are distributed dielectric multilayer stackreflectors. In alternative embodiments, materials other than dielectricmaterials may be employed for some or all of the layers, e.g.semiconductor materials.

[0209] In the embodiments described above, the MRBR is a distributeddielectric multilayer stack reflector. In an alternative embodiment, analternative MRBR may be formed by disposing two DBR mirrors on opposingsurfaces of an intervening layer, such as a piezoelectric (electrooptic)layer. Such a reflector will form an etalon, which has a reflectivityprofile having a plurality of reflectivity peaks. Such a reflector maybe referred to as a DBR-piezoelectric-etalon reflector. The reflectivityand spacing of the peaks depend on the reflectivity profiles of the twoDBRs and the intervening piezoelectric layer, including the refractiveindex and thickness of the intervening piezoelectric layer. For example,comparatively low reflectivity (e.g., about 80%) dielectric DBRs may beemployed, to give rise to etalon reflectivity peaks well above 99% inreflectivity. The characteristics of such a reflector may be selected,for example, so that the reflectivity peaks fall on ITU gridwavelengths, and may be integrated into, e.g., a two-section VECSEL. Thegain spectrum may cover several of these reflectivity peaks, and may beadjusted to select lasing at one of them. Additionally, a voltage can beapplied across the piezoelectric layer to change its index of refractionand to shift the reflectivity peaks. This may be used, for example, tofine-tune the lasing wavelength when the laser is locked onto a givenone of the reflectivity peaks. Alternatively, the characteristics ofsuch a DBR-piezoelectric-etalon reflector may be selected, for example,so that they are widely spaced so that only a single reflectivity peakfalls within the gain spectrum. In this case, a voltage across thepiezoelectric layer may be varied to shift (in wavelength terms) thereflectivity peak closest to the gain spectrum maximum, to tune orchange the lasing wavelength.

[0210] In the present application, a “non-section-112(6) means” forperforming a specified function is not intended to be a means under 35U.S.C. section 112, paragraph 6, and refers to any means that performsthe function. Such a non-section- 112(6) means is in contrast to a“means for” element under 35 U.S.C. section 112, paragraph 6 (i.e., a“section-112(6) means”), which literally covers only the correspondingstructure, material, or acts described in the specification andequivalents thereof.

[0211] Some embodiments or aspects of the present invention can also beembodied in the form of computer-implemented processes and apparatusesfor practicing those processes. The present invention can also beembodied in the form of computer program code embodied in tangiblemedia, such as floppy diskettes, CD-ROMs, hard drives, or any othercomputer-readable storage medium, wherein, when the computer programcode is loaded into and executed by a computer, the computer becomes anapparatus for practicing the invention. The present invention can alsobe embodied in the form of computer program code, for example, whetherstored in a storage medium, loaded into and/or executed by a computer,or transmitted as a propagated computer data or other signal over sometransmission or propagation medium, such as over electrical wiring orcabling, through fiber optics, or via electromagnetic radiation, orotherwise embodied in a carrier wave, wherein, when the computer programcode is loaded into and executed by a computer, the computer becomes anapparatus for practicing the invention. When implemented on a futuregeneral-purpose microprocessor sufficient to carry out the presentinvention, the computer program code segments configure themicroprocessor to create specific logic circuits to carry out thedesired process.

[0212] The present invention, therefore, is well adapted to carry outthe objects and attain the ends and advantages mentioned, as well asothers inherent therein. While the invention has been depicted anddescribed and is defined by reference to particular preferredembodiments of the invention, such references do not imply a limitationon the invention, and no such limitation is to be inferred. Theinvention is capable of considerable modification, alteration andequivalents in form and function, as will occur to those ordinarilyskilled in the pertinent arts. The depicted and described preferredembodiments of the invention are exemplary only and are not exhaustiveof the scope of the invention. Consequently, the invention is intendedto be limited only by the spirit and scope of the appended claims (ifany), giving full cognizance to equivalents in all respects.

What is claimed is:
 1. A planar lightwave circuit (PLC) module forconditioning light output from a tunable laser designed to generatelight at a target wavelength, the PLC module comprising: a substrate; aprimary waveguide embedded in said substrate, said primary waveguidehaving an input end for receiving light from the tunable laser and anoutput end for outputting said light; at least a first secondarywaveguide embedded in said substrate, said first secondary waveguidereceiving a first portion of said light from the tunable laser; and afilter having a passband centered on the target wavelength and coupledto an output of the first secondary waveguide to receive said firstportion of light, wherein said filter is adapted to generate a signalrelated to the intensity of said first portion of light in the passbandcentered on the target wavelength.
 2. The PLC module of claim 1, furthercomprising a second secondary waveguide that receives a second portionof said light from the tunable laser and a power monitoring photosensorcoupled to an output of the second secondary waveguide to receive saidsecond portion of light, wherein said photosensor is adapted to generatea signal related to the intensity of said second portion of light. 3.The PLC module of claim 2, wherein the power monitoring photosensorcomprises a photodiode.
 4. The PLC module of claim 2, wherein saidsecondary waveguides each receive a respective portion of said lightfrom the tunable laser by direct or indirect evanescent coupling fromsaid primary waveguide.
 5. The PLC module of claim 1, wherein saidsubstrate is a silica or silicon substrate and the waveguides arepatterned silicon oxide waveguides embedded in said substrate.
 6. ThePLC module of claim 1, wherein the target wavelength is one of aplurality of different target wavelengths.
 7. The PLC module of claim 6,wherein the filter is a tunable filter the passband of which can beselectively centered on any of the plurality of target wavelengths. 8.The PLC module of claim 6, wherein the filter is a multiple-outputfilter having a plurality of filters, one for each of the plurality oftarget wavelengths, each of said filters having a passband centered on arespective one of the plurality of target wavelengths and adapted togenerate a signal related to the intensity of said first portion oflight in the respective passband of said each filter, whereby saidmultiple-output filter provides a plurality of output signals related,respectively, to the intensity of said first portion of light inpassbands centered on each of the plurality of target wavelengths,respectively.
 9. The PLC module of claim 8, wherein said multiple-outputfilter comprises a reflectively coupled zigzag waveguide.
 10. The PLCmodule of claim 8, wherein said multiple-output filter comprises: aplurality of substantially identical distributed dielectric multilayerstack filters mounted in the substrate, each multilayer stack filterhaving a passband determined in part by the angle at which filteredlight impinges on said filter; a plurality of secondary filterwaveguides, one for each of the multilayer stack filters, each of theplurality of secondary filter waveguides receiving light from said firstsecondary waveguide and patterned in the substrate so as to terminate ata unique angle with respect to its corresponding multilayer stack filterso that each multilayer stack filter has a passband centered on arespective one of the plurality of target wavelengths.
 11. The PLCmodule of claim 1, wherein said primary waveguide comprises aconditioning device between its input and output ends for conditioningsaid light so that the primary waveguide provides, at its output end,light from the tunable laser conditioned by the conditioning device. 12.The PLC module of claim 11, said conditioning device is one of asemiconductor optical amplifier and a modulator.
 13. The PLC module ofclaim 1, wherein said first secondary waveguide receives said firstportion of light by direct or indirect evanescent coupling from saidprimary waveguide.
 14. A system for conditioning light output from atunable laser designed to generate light at a target wavelength, thesystem comprising: a planar lightwave circuit (PLC) module comprising: asubstrate; a primary waveguide embedded in said substrate, said primarywaveguide having an input end for receiving light from the tunable laserand an output end for outputting said light; at least a first secondarywaveguide embedded in said substrate, said first secondary waveguidereceiving a first portion of said light from the tunable laser; and afilter having a passband centered on the target wavelength and coupledto an output of the first secondary waveguide to receive said firstportion of light, wherein said filter is adapted to generate a filteroutput signal related to the intensity of said first portion of light inthe passband centered on the target wavelength; and a processor forgenerating, based on said filter output signal, a control signal, toadjust the lasing wavelength of the tunable laser to achieve or maintainthe target wavelength.
 15. The system of claim 14, further comprisingthe tunable laser.
 16. The system of claim 14, further comprising drivercircuitry for receiving said control signal and for generating, inresponse to said control signal, analog signals to control the lasingwavelength of the tunable laser.